Chapter 4 The human eye: a biological camera

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The human eye is an optical instrument offering incredible performance: not only is it capable of forming an image focused on its sensor (the retina) but it also has the abil-ity to focus at different distances and work under a wide range of light levels boasting

optical quality that is optimized for its functions. This is why for years it was the weak point of Darwin’s theory of evolution, as he himself acknowledged:

How is it possible that simply through the process of natural selection, such a perfect biological optical instrument had been created?

Yet the ability to see does not belong solely to the eyes; rather, it is a complex process (Figure 4.1) that occurs throughout differentiated phases where visual information is per-ceived, recognized, transformed and processed, over three stages: optic, retinal and neuro-nal. Our eyes form the image of the outside world, the brain interprets the image from each eye “in real time” and vision is the incredible result of highly coordinated teamwork.

The eye, the biological optical training system of imagesThe first stage of the visual process is the optical stage, where the eye is the main protago-nist when collecting light from objects in our environment. If a distant object is observed, we can consider that the propagation of light is rectilinear and parallel to the position of our eye and that it travels through the air, a homogeneous medium with an index of refrac-tion with a value of 1. When light enters the eye, it changes from the air to an aqueous medium (with an approximate refractive index of 1.334), in which its transmission speed changes, producing the phenomenon of refraction. Furthermore, the eye is formed by dif-ferent tissues that include transparent biological structures that function as converging lenses (cornea and lens, the latter is a variable power lens), transparent fluids that pro-vide nutrition and support (tear, aqueous humour and vitreous humour) and a diaphragm capable of offering openings adapted to ambient light (iris), which change the trajectory of beams of light by gathering them in the position of our retina in the same way as the lenses of a camera project the image on the digital sensor, as shown in Figure 4.1.

Finally, our eyes are wholly protected by the sclera, eyelids and bones of our eye orbit. This specialist design responds to the requirement to capture almost infinite information from the outside world and focus it on our biological sensor, the retina. As can be seen, this layout can resemble that of a camera (Figure 4.2), with its lens and eyepiece, sensor and housing, but let’s see more of its components in detail.

The cornea provides the greatest part of the eye’s refractive power, since it contributes approximately 2/3 of the eye’s total power. This major contribution is due to the shape of the corneal surface (convergent meniscus type lens) and the difference in the refractive

The human eye: a biological camera

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106 Discovering Light: Fun Experiments with Optics

index between the air and the cornea (with water-like value); it is the first medium (together with the tear film) with which the light is found at its entry into the eye. The lens is the second lens that forms the optical system; it is a converging biconvex lens that contributes the remaining third to the refractive power of the eye (about 20 diopters in the unattended state; later, we will delve into greater detail on the accommodation, the eye’s autofocus). Both the cornea and the lens act to transmit and focus light optimally on the retina, where transparency in both tissues is a critical requirement. For this, the cornea has a highly organized structure, while the crystalline lens has different interfaces and proteins (called crystalline proteins) distributed regularly as in onion layers, meaning that simple optical systems cannot be considered. This will be seen more closely in Experiment 4.1.

The iris is found between the cornea and the lens and is a diaphragm that, thanks to its dilator and sphincter muscles, modulates the entry of light into the eye. The size of the aperture delimited by the iris, which we call the pupil, varies automatically depend-ing on the intensity of light that reaches the eye (acting as the variable diaphragm of a photo camera).

These are the optical elements of the eye; they are not artificial lenses or the mechani-cal structure of a camera, but they are tis-sues that need support, protection and nutrition. Alongside this, we have three dif-ferent types of transparent fluids through which light propagates (tear film, aqueous humour and vitreous humour) and support tissues rich in collagen, fibers and blood vessels that are out of the light’s path (the sclera, the eyelids, the conjunctiva, the cili-ary muscle and the choroid), vital for the maintenance of the visual system.

Figure 4.1 The process of human vision: optical stage, retinal stage and neuronal stage and its analogy with a camera, sensor and processor system.

Source: María Viñas and Camilo Florian Baron.

Figure 4.2 The eye as a biological photo camera.

Photography: Tinypic.

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Chapter 4 The human eye: a biological photo camera 107

Retina: the magic of lightAround 90% of the sensory information we receive from our environment is visual, and its input is produced through the retina, the structure of the back of our eye. The retina is located in the position where the light rays that have entered the eye are focused and is the easily observable part of the central nervous system, since it has neuron-like cells. These specialized cells convert the light signal into an electrical impulse, triggering the second stage of the visual process: phototransduc-tion. Fascinating, isn’t it? But what happens when the light reaches the retina?

Santiago Ramón y Cajal (1852−1934), founder of modern neurobiology, was the first to accu-rately show the nervous system, the existence of neurons and the connection between them, and the organization of cells in the different layers of the retina (the structure represented in some of his drawings, such as Figure 4.3).

Contrary to what we might imagine, the light refracted by the cornea and the lens does not reach the photoreceptors directly but must firstly pass through the neurons to which the photoreceptors are connected. The explanation is that the retinal neurons are transparent, like almost everything in our eye, and the photore-ceptors with their opsins consume a lot of energy to start the visual process; thus, they need to be connected to their power supply: the pigmen-tary epithelium, which is not transparent but has a molecule that absorbs light and does not let it go further. The pigmentary epithelium is not responsible for initiating the visual process; it only acts as a barrier to light. It is somewhat odd, but years of evolution have determined this process as one of the most fascinating and spe-cialized aspects of the human body.

In the human retina, there are two types of photoreceptors that, due to their shape, receive the names of cones and rods. The rods are spe-cially adapted to night vision, as they are very sensitive and can detect very low levels of light, on the order of around a few photons (or a few lights). As there is only one type of cone, we are unable to distinguish colors in low-light condi-tions, when only the rods are active. The rods are responsible for daytime vision, when the light levels are higher, and therefore, they are not as sensitive as the cones. Being smaller, they can be packaged with higher density (more “pixels” per

Figure 4.3 The retina is the inner layer of the eyeball, it has a complex structure composed of ten different layers of cells. It was described in detail by Santiago Ramón y Cajal in 1900.

Source: Retinal structure system belonging to the manuscript Study of the olfactory, optical and auditory brain centres and continuity relationships that have nerves of the same name with them in the human species and vertebrates, Santiago Ramón y Cajal, Cajal Institute, CSIC.

Did you know…?

Ramón y Cajal made a major contribution to the knowledge of the histological structure of the retina by means of the Golgi staining method and, thanks to his skill in drawing, he discovered the ins and outs of the different types of cells that make up the retina: photoreceptors, bipolar cells, horizontal cells, amacrine cells and ganglion cells. Although the detail of his drawings is incredible, thanks to the advancement of microscopes and the latest high-resolution imaging techniques, we have been able to obtain genuine photographs of the retina with very high quality and advance somewhat further into his knowledge (Figure 4.4).

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unit area), providing greater-detail resolu-tion than the rods. There are three different types of cones, sensitive to different areas of the visible spectrum, as we will see later, which allows us to distinguish colors in daylight conditions.

Another of the most fascinating features of the retina is how the photoreceptors are distributed. We have said that cones help us see in great detail. Well, there is an area of the retina, slightly off-center towards the nasal side, called the fovea, which consists only of cones, meaning that it is the area of the retina that receives the greatest amount of light. The fovea is known as the point of greatest visual acuity and is the point with which we gaze when we want to see something specific. The density of the cones decreases as we move away from the fovea, and more and more rods begin to appear until an area arrives, on the

periphery, where the cones cease to have a presence and are replaced solely by rods.If the ocular optical system resembles a camera, we can say that the retina is our digital

sensor. However, an important difference between a camera’s sensor and the eye’s retina is that, while the camera’s sensor is limited to representing the point-to-point image (in each pixel) so that it can then project on a screen, the retina also analyzes the image that arrives, and instead of transmitting a point-to-point representation of the image to the brain, it transmits prioritized information to the brain such as, for example, the edges of the objects in the image and orientation of objects.

For this reason, the retina not only contains light sensitive cells (photoreceptors), which transform into electrical activity the light energy, but are also connected to a network of neurons that process said electrical signal that then leaves the eye through the neuronal axons that form the optic nerve. The point of the retina where these neuronal axons meet to form the optic nerve is called the optic disc (Figure 4.5). In this area of the retina, there are no photoreceptors that perceive light, which is why it is called the blind spot. This is located about 15º towards the temporal zone with respect to the fovea and, although in that area we do not see, we do not realize why the brain “fills” that part of the image. In Experiment 4.2, we will see this part.

Finally, the light signal travels through the visual pathways and is encoded according to its different characteristics (detail, color, movement) until it reaches the visual cortex, in the occipital lobe of the brain, responsible for processing the visual information.

But is the eye a good optical instrument?As can be seen, the eye has many similarities with a camera, with its lenses that focus on a receiving screen or the diaphragm that controls the amount of light. Although it must be said that the eye has much more sophisticated mechanisms than those of a camera, it is also true that the quality of the image that forms on the retina is not quite perfect. Let’s do a test: look at a small and somewhat distant point of light, such as an LED that emits any appliance in your home. We know we should see a point. Close one eye. Don’t you think that the point seems to have legs and halos, as if it were a kind of flea instead of a point? Now try looking with the other eye. That little flea is different! From this experience, we can draw two conclusions: if the image that gives us the eye of a point is not another point,

Figure 4.4 Image of a confocal microscope of the cones (in red) and rods (in green), photoreceptors of the retina.

Photograph: Courtesy of Robert Fariss, National Eye Institute (NIH).

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it means that the eye has certain imperfections. Furthermore, these imperfections are dif-ferent between the two eyes.

Indeed, the eye is not a perfect optical system. The differences between the images of the real point of the LED and the one that you have seen with each of your eyes are called aberrations, and they can be measured, quantified and their influence studied, although today it is not possible to correct them with glasses. In addition, it must be kept in mind that there are other phenomena such as diffraction or dispersion that greatly affect the final quality of the image.

Helmholtz (1821−1894) was a famous 19th-century scientist, with studies in numerous fields, who developed important knowledge in optics. His phrase was famous, saying, “It is not an exaggeration to say that if an optician wanted to sell me an instrument that had as many defects as those of the eye, it would be justified to reprimand his lack of care in the most energetic way and return it.” However, Helmholtz knew very well that the defects that the eye has are not as noticeable as in any other instrument. Why? As you have been

Figure 4.5 Diagram of the retina’s blind spot.

Source: Henry Gray (1825−1861), Edward Anthony Spitzka (1876−1922), Anatomy, descriptive and applied (1913); Philadelphia, New York, Lea & Febiger, Open Knowledge Commons.

Did you know…?

It was a seventeenth-century French doctor who, for the first time, while dissecting a human eye, observed that the optical disc lacked photoreceptors, so he assumed that it must be a blind spot. Sometimes, it is important to know if there is any part of the retina that is not functioning properly. For this, a test called campimetry is used where, while the patient gazes at a fixed point, lights are presented in different positions of the visual field to see which ones they see. The blind spot is precisely a control system since, if a light is projected that should fall into the blind spot and the patient sees it, it is not testing correctly.

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told in this chapter, the visual process does not depend only on the eye or end at the ret-ina. It is the brain that is responsible for interpreting all the information it receives through the retina and is responsible for compensating some of these imperfections or adapting to them, so that in everyday life we are not fully aware of them. The brain is adapted to the visual quality of each one and, when that quality changes, as when we wear new glasses, for example, it must re-adapt to the new quality of the images it receives.

The brain: the last stage of the vision processIn the previous two sections, we have seen two stages, the first, which corresponds to the optical process of light transmission, and the second, which is known as the process of transforming light stimulation upon nerve impulse. Phase three includes the transmis-sion of nerve impulses to the visual cortex, in the occipital lobe, located in the posterior cerebral fossa. And finally, the fourth phase is the interpretation of the information in the cerebral cortex in real time, completing the visual process.

The brain is perhaps the most complex biological structure in evolutionary history. However, visually speaking, it is surprisingly easy to fool, since it has its limitations. The brain samples with great speed and resolution the information that is critical, which has more con-tent, but limits the rest of the information to a filling process, which it does not consider so

Figure 4.6 Red eyes when illuminated by a photographic flash due to blood vessels of the choroids.

Photography: PeterPan23, Wikimedia Commons.

Did you know…?

The “red eyes” that we often see in the photographs are due to the light that, reflected in the back of the eye, comes out through the pupil. The pupil normally appears black because most of the light that enters the eye is absorbed and does not come out again. However, when we use an intense and very-short-duration flash, as is the case with the photographic flash, the choroid has no time to contract, and we see the light that is reflected in the retina and leaves the eye again, filling the dilated pupil. The reddish tone is due to the color of the blood vessels found in the retina.

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relevant. That is, our brain selects the information it wants to process and frees us from the rest; this is an effective strategy in terms of neural resources. Imagine the energy resources we would consume if we had to process all the information coming from each of our senses.

This active process of the reconstruction of our visual perception is very similar to our reality, but not completely. And when there is a lack of correspondence between percep-tion and reality, optical illusions come into play. The basis of optical illusions is psychol-ogy, and they can be classified into several groups: on the one hand, we have those that are due to physiological phenomena, which depend on the physical response of the eye to a stimulus (for example, the bleaching of flash photoreceptors of a photo camera, or the disappearance of an object whose image falls on the blind spot of the retina or the few details that our objects that are on the periphery of the scene have) and on the other hand, we have the optical illusions that are due to psychological phenomena, which are based on the geometric perspective and are related to the association of ideas during learning. Optical illusions are a combination of physiology and psychology that involve the entire visual process (from the eye to the brain), and its final result is deception. Wizards learned their use centuries ago. We will explore this in Experiments 4.3 and 4.7.

The accommodation: a natural autofocus systemOnce the visual stages are completed, the unique concepts of the visual system will be described. One of them is our astounding ability to focus distant and near objects auto-matically and without losing detail; this wonderful property is down to the lens.

As we saw in Chapter 3, when we have a lens with fixed refractive power, the position of the image produced by the lens depends on the position of the object. When the object is too far away, the image will form in the focal plane image, so we can place our sensor (the retina, in our case) at this point to obtain a focused image. If we approach the object, the vergence

Figure 4.7 Salvador Dalí’s painting “Naked Gala looking at the sea that President Lincoln appears at 18 meters”, in which the double image and the blur (left) are dabbled with. Also shown is the optical illusion of an impossible cube. Our brain is able to see a complete cube, even if some of the information is wrong (right).

Photography: Dalí, Flicker (left); Pixabay (right).

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of the light that reaches the camera’s target from the object increases. This means that to maintain the same image distance as in the previous case, for the image to form in the same position, the image vergence must be the same. This is only possible if we increase the refractive power of the lens of our cam-era: L′= L + F; if L′ = 1/l′ is constant and L = 1/l increases when the distance / is reduced, then F must increase.

In the case of a camera, this is achieved by focusing the lens. By focusing the lens via rotation, we actually change the relative position of the lenses that form it, increas-ing the total power of the lens system. In the case of the eye, to focus closer objects on the retina we also need to increase its refractive power, although the mechanism, which is called accommodation, is different from that of cameras. In the case of the eye, the increase in refractive power is achieved by increasing the power of the lens, mainly by increasing the curvature of its anterior face (we saw in Chapter 3 that by increasing the curvature of a curved refractive surface, its power increases)

(Figure 4.8). The accommodation process is continuous; the eye accommodates or does the opposite every time we look at a new object. However, with age the lens becomes less elastic, and we gradually lose the ability to accommodate (at 40, we can hardly read this book without an optical aid, modern glasses or contact lenses; who knows if in the next few years we will have a different type of autofocus lens).

Focusing errors in the eye: why do we need glasses?During our eyes’ development, we need the focal point of the eye’s optical system to coin-cide exactly with the position of the retina, requiring a high degree of coordination between the cornea, the lens and the length of the eye. An eye that has the ability to sharply focus the image on the retina is said to be emmetropic. However, sometimes the eye is not able to sharply focus the image, presenting some kind of error and reducing the optical quality of the eye. That means that the size of the eye is not adjusted to the power of the eye’s optical system, triggering near-sightedness or farsightedness, or that the lenses themselves have some kind of irregularity, resulting in some astigmatism.

If the eye is too long or has a convergent refractive power, the image of an object is formed in front of the retina and we say that the eye is myopic. In this case, the light beams are focused in front of the retina, but if we continue their propagation to the retina we see how they continue to diverge and form a blurred spot in the position of the retina (Figure 4.9). On the contrary, if the eye is too short or its refractive power is not enough to make light beams converge on the retina, we say that the eye is long-sighted. In this case, the rays reach the convergent retina but point to a spot behind the retina (Figure 4.9).

A myopic eye can be corrected by divergent lenses. These decrease the convergence of the eye the amount necessary for the image to form on the retina. When we say that an eye is myopic of −3.0 D, what is meant is that the power of the corrective lens needed is −3.0 D, that is, divergent. On the contrary, a long-sighted eye does not have enough

Did you know…?

Many optical illusions have a significant impact on our daily lives, such as Helmholtz’s theory of illusion. This theory is applied both in the world of fashion as well as in decoration, giving a broader perspective of space. The choice of stripes for the clothing of prisoners is no accident, as it makes them more visible and facilitates their identification in a crowd in case of escape. What is more, the stripes constitute a psychological punishment, since in the Middle Ages these prints characterized prostitutes, jesters and other social outcasts.

Benham’s disc, which we will see in Experiment 4.7, was invented by Charles Benham, a journalist and inventor fond of toys, and at first it was sold as a simple toy. However, it is currently used by some specialists as a method to diagnose some eye diseases, especially cases of optic neuritis.

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refractive power to form the image on the retina, and we need to contribute more conver-gent refractive power. Therefore, when we say that an eye has hypermetropy of +3.0 D, we know that this is the lens that corrects its refractive error, focusing the image on the retina. This will be seen in Experiment 4.4.

The near-sighted or hypermetropic have the focal point in front of or behind the retina because the power of the eye or its length are not correct. However, if there is some kind of irregularity or decentralisation in the cornea or lens, then a single focal point is not formed, but different foci are presented for different orientations. This is known as astig-matism, and its correction would be done with cylindrical or toric lenses (Figure 4.9).

Why do we have two eyes? Binocular, three-dimensional (3D) vision and visual field Binocular vision is the integration of the sensation produced by the light stimuli that reach each eye in a unique perception (Figure 4.10).

Having two eyes is not synonymous with binocular vision. The chameleon, for example, has two eyes that move independently and yet has no binocular vision. For this to take place, both eyes are required to move in a coordinated manner, so that the final impres-sion of the outer space is unique. The fundamental advantage of binocular vision over monocular vision is the possibility of deep vision or stereoscopic vision. We will check this phenomenon in Experiment 4.5.

Stereopsis is a phenomenon of perception that provides valid and accurate information on the relative distance of an object from another. This concept is usually confused with

Figure 4.8 Illustrating the accommodation process.

Photography: Pablo Pérez Merino.

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the term depth perception. However, these are totally different concepts, since there are patients with monocular vision who can perceive relative distances between objects in space and, nevertheless, do not have stereopsis. Binocular vision, together with retinal dis-parity, that is, the difference between two images in the retina (because the two retinas do not have the same vision stimuli given the location of the eyes) is the basis for the creation of images in three dimensions (3D) on flat surfaces, used in different space exploration projects (Figure 4.11).

The binocular field of vision contains the visual field of the two eyes, without moving the head, and this extends through a region that exceeds 180º horizontally, but there are regions that are invisible to each of the two eyes due to the nose.

Did you know…?

We are all born with hypermetropias, and as we grow, the size of the eye is synchronised with its refractive power so that the image is focused on the retina, and we do not have refractive errors. This process is called emmetropisation, and once again the eye surprises us by being able to determine if it has to reduce or increase its growth rate. However, lifestyle changes seem to affect this process, causing an epidemic of myopia that is beginning to be a problem in countries like Singapore, where 90% of the population has myopia.

Figure 4.9 Diagram of the different refractive errors, when observing a scene and with its corresponding corrections.

Photography: in house.

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How many images per second can the human eye really see? MovementWhen moving an object in front of our eyes, a succession of its images is formed in different regions of the retina, giving the sensation of movement. However, the sensation may also be noted when it is produced by objects that are at rest and that appear and disappear in a timely manner. This sensation is called appar-ent movement, as opposed to real movement. We will see this in Experiment 4.6.

In order for the brain to distinguish between two images that form in two differ-ent regions of the retina, these images have to be formed at well-defined time intervals. Hence, the concept of the temporal resolu-tion of the human eye, and the relationship

Figure 4.10 Horizontal and vertical visual field.

Photography: OpenStax College, Wikimedia Commons.

Figure 4.11 Two spacecraft launched by NASA within the STEREO mission (Observatory of Sun and Earth Relations).

Photograph: NASA.

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with a basic concept in film or video games, the FPS (frame or frames per second), that is, the series of images we are able to spin per second to observe a sequential series as a continuous series. In other words, a sin-gle moving image, which, after all, is what humans can envisage.

Therefore, the exact exposure is 24 frames per second, that is, a film in which 24 frames per pass in front of the projec-tor for each second of exposure to light. The characters are observed as people in real situations with real movements, when in fact we have deceived our eyes and our minds, an example of which can be seen in Figure 4.12. However, if these same frames were exposed at a different speed, our view would perceive those same situations

as not real. The proportion is as follows: the greater the number of frames exposed per second is, the greater the sensation of slowness; and the smaller the number of frames exposed per second is, the greater the sense of speed. For example, at 36 frames per sec-ond, the human eye perceives a situation in what is now known as slow motion, while the same image, or the same frames, if reproduced at a number less than 24, for example, at 18 frames per second, results in us being able to perceive those same situations in rapid motion, that is, both the characters and the activities they develop. The only detail that has changed is that fewer exposures per second of the same previous frames pass in front of the projector.

How has the organ of sight developed in other living beings?The animal kingdom encompasses millions of species, and more than 95% of these share one characteristic: the power of sight. However, there are no two species that see the world in the same way. There is a great variety of eyes in the animal world; there are eyes with different aspects, sizes and settings. This is because animals have different lifestyles that have made them evolve to ensure their survival.

Animals’ eyes are an excellent example of convergent evolution. The eyes of the dif-ferent animals throughout the current planet did not evolve from the same ancestral eye. In fact, the eyes have evolved in different lineages, but, extraordinarily, evolution has used the same basic genes for the creation of the eyes of creatures as different as the fly, the squid or the human being. At the beginning of the Cambrian period, 544 million years ago, the animal kingdom experienced a transitional stage, as new species appeared, including marine ancestors of dinosaurs, elephants and humans. These spe-cies were larger and more mobile than their predecessors, developed combat weapons and, most crucially, their eyes. The eyes of the trilobites are the first to appear in the fossil record of compound eyes. Trilobites are animals with joint limbs, which currently cover crustaceans, insects and spiders, among others. Observed closely, the compound eye consists of two round structures on the head, in which small bodies can be seen cir-cular in a series of rows, each of which contains several of these circles, which are actu-ally lenses (Figure 4.13).

The next step in the evolution of the eye can be imagined in the following way: if we have a sheet of cells sensitive to luminosity and fold that sheet in a “U” shape, we would

Figure 4.12 The flutter frequency of this humming-bird coincides with the frames of the camera, so that it seems to be suspended in the air.

Photograph: Photography: MaxPixel ,https://www.maxpixel.net/Bird-In-Flight-Colorful-Humming-Bird-Wildlife-2507644.

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obtain a concave figure called an eye in a glass. If we continue folding gradually, the cup closes on itself leaving only a hole at the top, which provides the eye with a mechanism similar to that of a pinhole camera. The nautilus, a genus of cephalopod molluscs, has eyes of this type (Figure 4.14), simple, that furnish it with blurred vision of the environment and with scant details, but it allows them to determine certain simple forms.

The key to obtaining a clearer image that provides good vision is found in the lenses. The better the lens is, the better the image obtained will be. Natural selection has continued working generation after generation, imposing in each phase small improve-ments on different characteristics, such as the curvature, the transparency of the lenses and ocular medium, until finally a degree of evolution like that of the human eye was reached.

However, we find in nature many examples of animals with more surprising eyes than ours; for example, we can highlight the primate called the tarsier. Its huge eyes are fixed on the skull and cannot rotate in its orbits, a disadvantage that this animal compensates with a neck that rotates 360º. As its eyes are heavier and bigger than its brain, the tarsier

Figure 4.13 Eyes of animals with different complexities (from left to right): human, nautilus and trilobites. In the upper part, the animal; in the middle, an experimental setup; and below, a ray tracing.

Sources: Sofiezborilova, Pixabay (top left); Skeeze, Pixabay (above centre); Adolfo-atm, Pixabay (top right); in house (centre); Clara Benedí (below)

Did you know…?

The film industry is testing the 48 FPS format, twice the traditional rate of 24 FPS. The first film that was shot at 48 FPS was The Hobbit (Peter Jackson). At first it may seem that the movements are excessively fast and artificial, but this is only because we are accustomed to 24 FPS. With 48 FPS, each frame is exposed to light for less time because more images are taken per second, so the sharpness of each frame is higher.

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has very sharp sight and excellent night vision. Also interesting is the case of the goblin fish, which incorporates in its vision system a mirror that allows it to see up and down at the same time.

BibliographyAtchison, D.A. AnD smith, G. (2000): Optics of the Human Eye, Oxford, Butterworth-Heinemann.DiAz, R. (2013): Art, magic and illusion: optical illusions in art and other visual productions, Madrid,

CSIC.KAhle, W. AnD FoRtscheR, m. (2015): Anatomy Atlas with Clinical Correlation, Thieme Medical Pub;

7th edition. mAcKniK, s., mARtinez-conDe, s. AnD BlAKeslee, s. (2011): Sleights of Mind: What the neuroscience of

magic reveals about our brains, Profile Books.Puell, m.c. (2006): Óptica Fisiológica: el sistema óptico del ojo y la visión binocular. Universidad

Complutense de Madrid, Madrid.schWARtz, s.h. (2013): Geometrical and Visual Optics, New York, McGraw Hill Professional.stRAtton, G.m. (1986): “Some Preliminary Experiments on Vision without Inversion of the Retinal

Image,” Psychological Review, 3 (6), pp. 611-617.VAlenzuelA, m. (2008): Anomalías en la visión del color. Publicatuslibros. https://docplayer.

es/19554919-Anomalias-en-la-vision-del-color.html.

Figure 4.14 The structure of the eye of the octopus is very similar to that of the human eye.

Photograph: Edmondlafoto, Pixabay.

Did you know…?

The structure of the eye of the octopus is very similar to that of the human eye, except the structure of the retina, which is reversed (the light-sensitive cells are closer to the lens than the rest of the cells of the retina). This indicates that this type of eye evolved very similarly to ours, despite following a vastly different evolutionary path.

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60 min (+)

Experiment4.1

How Does the Human Eye Work? And that of Other Animals?

Anatomy of the eye.

The sight process is complex and needs three basic elements: eye, brain and light. Without light, we are unable to see. However, in many cases it is the optics of the eye that determines the type of vision.

In this experiment, you can mount two different eye models, in which various opti-cal elements are involved: a pinhole and a lens. Of course, the quality of the image that reaches the retina is not the same.

Try for yourself!

Figure 4.1.1 Formation of a image in the eye and in a camera.

Source: In-house

ProcedureThe nautilusThe nautilus has the simplest visual system you can imagine: a simple pinhole that regu-lates the amount of light that enters, and no lens. 1. Make a small hole in some cardboard. You can draw the animal on the cardboard! 2. Illuminate a slide so that the image passes through the pinhole. Place a white card-

board behind the nautilus to see how the image is formed.

The human eye 1. Take the transparent circular plastic and stick it to one end of the PVC pipe. 2. Glue the piece of onion paper over the clear plastic. This is where the image will be

projected, that is, the retina.

OBJECTIVES:Objective 1: Mount the eye of the nautilus.Objective 2: Build and identify the parts of a human eye and

compare them with the parts of a camera.

MATERIALS• Plastic ball• PVC or sturdy cardboard tube• Convex lens (magnifying glass)• Flashlight• Circular piece of transparent

plastic with a diameter somewhat larger than the PVC pipe

• Piece of onion paper the same size as the circular piece of plastic

• Craft knife, scissors, glue and adhesive tape

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4.1 Experiment: How Does the Human Eye Work? And that of Other Animals?

Figure 4.1.2 Nautilus eye diagram (left) and assembly of the experiment (right).

Source: in house. Photography: Eliezer Sánchez González / Scientific Culture (CSIC) / IOSA.

Figure 4.1.3 How to use the model eye (left). Looking through the model eye (right).

Photography: Eliezer Sánchez González / Scientific Culture (CSIC) / IOSA.

3. Cut the plastic ball in half, with the help of the craft knife, remember not to cut yourself! 4. Make a hole in the centre of each of the halves. The PVC pipe will pass through one

of the holes and the lens (magnifying glass) will be placed in the other. This lens will represent the cornea and lens ensemble.

5. Insert the PVC tube so that the plastic and onion paper are inside the ball. 6. Adjust the distance between the magnifying glass and the paper until a focused image

is obtained. 7. Join the two halves of the ball and fix them with sticky tape.

ExplanationLight enters the eye through the cornea (the magnifying glass in our experiment). The iris controls the muscles that alter the size of the pupil in order to adjust the amount of light entering the eye (in this case, the hole that simulates the pupil is of fixed size). After pass-ing through the pupil, the light reaches the lens (the magnifying glass), which focuses the light on the retina (the transparent circle covered with onion paper). The retina contains thousands of cells that are sensitive to light and help transform this into a neural message for the brain to interpret as an image. The eye is optically equivalent to the current camera: it has a lens system, a variable aperture system (the pupil) and a retina that corresponds to the photographic plate on which the images are projected.

The jumping spider (Salticidae) has one of the finest visual systems in the world of invertebrates. In total, it has eight eyes, six lateral and two in the front of its head, so that they provide a kind of peripheral vision. The main eyes are located in the front and allow it to locate prey and hunt them. These eyes have two lenses like our cornea, which make

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Experiment: How Does the Human Eye Work? And that of Other Animals?

4.1

a first filtering of the light, but then they have two tubular structures that reach a second lens. The two-lens system makes jumping spiders see the world through two small tele-scopes that can move and adjust. Just as we accommodate by changing the shape of our lens to change the focal length, the spider focuses with different layers of its retina. It has a succession of photoreceptors arranged in a kind of staircase, which allows the spider to clearly perceive objects at different distances depending on where you direct the stimulus, that is, one layer receives the focused images and the other receives them blurred. The more out of focus the image appears in this layer, the closer the eye is to the object. This detail allows the spider to calculate the exact distance it has to jump to catch its prey. The second, third and fourth layers are responsible for color vision, and the first layer works in high sharpness and shape perception. Combining the four layers, the spider has 3D vision.

TricksYou can paint the ball white so that it looks more like an eye, as well as the iris around the magnifying glass.

Let’s see what you have learned• How must the image on the retina appear?• The part of the retina we are observing, what would it be and why?

Related experimentsExperiment 3.6 From a shoebox to a camera

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Experiment

Retina, photoreceptors, retinal persistence, light, image formation

30 min (+)

What Happens in Your Retina?

The functioning of our body is based on numerous chemical reactions. For example, in the process of transforming the light signal that reaches the retina into a nerve impulse, a chemical reaction also occurs. With this experiment you can verify and

alter these processes.The retina is the part of the eye that receives the image information being viewed, just

like the sensor in a camera. So that all this information can be sent to the brain, the optic nerve is used, the “cable” that communicates between both organs. The junction zone between the optic nerve and the retina lacks photoreceptors, light-sensitive cells, making it a blind zone. You don’t believe it? Check it out for yourself!

ProcedureExperiment with the light of a flashlight 1. With the help of the craft knife, cut

a simple figure on the cardboard (a square or a triangle).

2. Get into a dark room. 3. Place the flashlight behind the hole so

that the light passes through the cut-out area, without protruding from the sides of the cardboard. Keep the card-board upright by placing books on both sides, for example.

4. Keep the cardboard and flashlight an arm’s length from you.

5. Look at the bright hole for 30 seconds. 6. Next, look at a blank wall and blink

a few times. What did you see? What color is it?

7. Now look at the palm of your hand and again the wall. What’s the difference between the two?

Figure 4.2.1 Assembly of the experiment. The flash-light is placed behind the cardboard.


4.2 OBJECTIVES:Objective 1: Observe post-images.Objective 2: Verify the existence of the blind spot in the retina

and calculate, taking into account the distances, what angle the fovea and the blind spot form.

Objective 3: Check how the brain can form a single complete image even if we only see different pieces separately.

MATERIALS• Flashlight• Cardboard• Ruler• White sheet of paper• Sticky tape or masking tape

• Utility knife• Ballpoint pen• Long cardboard tube

(like kitchen paper)• Black cardboard

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Experiment: What Happens in Your Retina?


4.2

Experiment with a cross and a circle 1. Draw a cross and a circle on paper sep-

arated from each other by about 12 or 15 cm.

2. Place the paper about 15 cm from your eyes.

3. Cover your left eye and look at the cross with the right.

4. Right now, the cross is focused on the fovea and the circle in a peripheral area of the retina.

5. Slowly take away the paper while look-ing at the cross.

6. There will come a time when the circle disappears. This is because it is being projected onto your blind spot on the retina!

7. How far from your eye do you have to place the paper so that you stop seeing the circle?

Experiment with a tube with a slit 1. Cut a piece of black cardboard to form

a lid on one end of the tube. Make it bigger so you can stick it to the tube.

2. In the middle of the circle you just made, cut a rectangle 2.5 cm long and 3 mm wide (millimeters! It has to be a very thin slit).

3. Wink one eye and place the tube through the open end on the other. You cannot see much, but if you move the tube (with your head and body still), even if you only see small pieces of the image, in your head you see the complete image. Move the tube at different speeds and see what happens!

ExplanationWhat is happening in the first part of this experiment is that, by observing a bright light for a long time, the retinal sticks become saturated. The rods contain a protein called rho-dopsin, which is sensitive to light stimulation. When it receives light, it breaks, and in the dark it forms again. By stimulating the protein with such light intensity, the regeneration process takes longer, which becomes a few seconds of “blindness.” The saturation that produced in the experiment is localized, and that is why the shape (the triangle or the square) of the figure we have made in the cardboard is maintained. The image you see is called a post-image.

In the second experiment, as long as you keep your eyes fixed on the cross, it will be projected on the fovea, the point of maximum visual acuity, and everything around (in

Figure 4.2.2 Parts of the eye involved in the detection of the blind spot.

Source: in house.

Figure 4.2.3 Assembly of the experiment.


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Experiment: What Happens in Your Retina?4.2

this case, the circle) in the rest of the ret-ina. If you move the paper but keep your eyes fixed on the cross, it will continue to be projected on the fovea, while the projec-tion of the cross will move over the entire retina, until it coincides with the blind spot, where there are no photosensitive cells, and therefore it can no longer be seen. With a little trigonometry we can calculate the separation between the fovea, the area of greatest visual acuity, and the blind spot in the retina. The angle (α) in question is equal to that subtended by the circle with respect to the cross:

α = arctan 12 or 15cm/d

In the third experiment, the distance (d) is an approximation, since it can be measured from the vertex of the eye and not from the center of rotation, so keep in mind that this value is always going to be somewhat greater. In this experiment, what is known as visual persistence is tested. The eye and the brain both retain images for a small fraction of a sec-ond. When you move the tube quickly, the brain retains the fractions of the images long enough to form a complete picture of what you are seeing.

Tricks• You can do even more with the post-images. Cover your eyes for 10 minutes. After

that time, uncover them, open your eyes and observe a well-lit scene for half a second. Then close your eyes again. It will seem like even with your eyes closed, you can still see the scene.

• You can complicate the post-image experiment a bit by playing with both eyes. Repeat the same process, but this time observe the illuminated figure with only one of them, keeping the other closed. Try looking again at the wall, first with one eye and then with the other. You will see that this effect only occurs in the eye that has been exposed to the great intensity of light.

• In the second experiment, to make it easier for you to measure the distance from your eye to the paper, place positioning marks and place the paper on some support so that the displacement is more uniform.

Let’s see what you have learned• In the post-image experiment, when the one-eye experiment is performed, why is the

effect not transferred to the other?• Why don’t we usually realize the existence of blind spots?• At what angle is the blind spot of the optical axis approximately?

Figure 4.2.4 How to calculate the blind spot angle.


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30 min (+)

Experiment

Color, contrast

The perception of colors plays an important role in our life. In February 2015, the photo of a dress began to circulate on social networks and caused a stir on the inter-net. What was special about the dress? Apparently, it was a normal dress, but half of

the world’s population saw it as golden and white, and the other half saw it as black and blue. You can search for the photo of the dress on the internet.

In this experiment, we will see how certain factors affect how we perceive colors and how our brain can deceive us.

ProcedureExperiment with colored cards 1. Cut one of the orange cards and one of the blue cards in half. Do it so that you

take the blue of the darkest shade and the orange of the lightest shade. Glue two of the halves of different color with sticky tape and leave the part that is not taped upside down. This will be our new backdrop.

2. From the rest of the colors (except grey, black, white and those you used to make the background), cut out equal squares of each color and a rectangular strip (of the same colors).

3. Take two squares of the same color and paste them at the same distance in the background that you prepared in step 1. Each square must be of a different color. Repeat this step with all squares.

4. Ask a friend which color they think is lighter or darker. Join the squares with the rectangular strip of the same color and show that, despite the background, the color is the same!

4.3

Figure 4.3.1 Experiment with colored cards.


And You, What Color Do You See?

OBJECTIVES:Objective 1: Observe colors under different conditions and see

how our perception of them changes.

MATERIALS• Scissors• Glue and sticky tape• Grey translucent paper (or grey adhesive tape)• Ruler• Black and white cardboard• Translucent colored paper: yellow, purple, two shades of blue

and two shades of orange

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Experiment: And You, What Color Do You See?4.3

White, black and grey cardboard experiment 1. Cut out eight strips of black cardboard

measuring 1.50 cm wide and 20 cm long.

2. Glue the strips on the white cardboard forming horizontal strips, leaving a 1.50 cm gap between them.

3. Cut out eight strips 1.50 cm wide and 5 cm long from the translucent paper or grey tape. If you use adhesive tape, stick it on a piece of paper beforehand so you can move it more easily. Place four grey strips on the right side of the cardboard you prepared in point 2. Place them as four consecutive black stripes.

4. Place another four grey strips on the opposite side and at a different height than the previous ones (start higher or lower) on the white parts.

5. Do you think they are the same shade of grey? Move the grey strips and watch what happens.

ExplanationCones are responsible for processing color sight. We have three different types of cones and each of them is sensitive to a different color. This makes us distinguish many colors, but it also means we can confuse them. Therefore, if we place a light blue color on a dark blue background, it will appear brighter, since the cones that process this color are centred on the dark blue color that is in the background, and the rest of the cones are not that sen-sitive to that color. Something similar is what happens in the experiment using grey with black and white card. When changing the grey strips’ position, it can be seen that at some point they appear to be the same shade of grey (when the two strips are parallel on black or

Figure 4.3.2 White, black and grey cardboard experiment assembly.


Figure 4.3.3 Visual perception of colors in your experiments.


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Experiment: And You, What Color Do You See?


4.3

white at the same time); but, when interspersed, one between black strips and one between white strips, the shade of grey is remarkably different.

The black strips also appear darker in some areas and lighter in others. Knowing that the tone of grey and black is always the same, why is this? Well, it is due to how our eye processes light and dark information. Clarity is a property related to the amount of light that objects reflect, and this is processed in conjunction with the image observed; we are not able to separate one object from another. If we cover the black lines and only see white and grey, white reflects 80% of the light it receives, creating a very bright environment, so grey is affected. However, if we cover the white and only see the black, 5% of the light it receives is reflected, and the environment becomes very dark. Grey has the ability to reflect 10−70% of the light it receives, depending on its environment, so…now we under-stand it! This is known as proof of clarity.

Tricks• Try changing the backgrounds, put darker or lighter colors to see which background

causes what you place on top is perceived as lighter or darker. The contrast also works in reverse: different colors may look the same.

• Try to take two similar but differentiable colors with the naked eye and find a back-ground where both look the same.

Let’s see what you have learnedWhy does the background on which we see the object affect the color of which you see the squares?

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Experiment

Image formation, refractive defects, retina1 h (+)

Why Do People Wear Eyeglasses?

4.4

The size of the eyeball and the shape of the cornea have parameters that allow the parallel rays that reach our eyes to

focus on the retina with sharpness.When one or more of these parameters do

not comply with the rule, the images that form on the retina are blurred. This is what is known as refractive defects, as was seen in Figure 4.9. Let’s experiment with myopia and farsightedness!

Procedure 1. Cut a hole in the middle of the bottom

of the black cardboard. Paste the image of the letter into this hole.

2. Place the magnifying glass behind the black cardboard at a distance not far away.

3. In the same line of sight, place the fish-bowl full of water and glue the white cardboard on the side opposite to the one closest to the magnifying glass. This will be the retina of our experimental eye.

4. Once all the elements are located in a straight line, light up the image with the flashlight to form the image on the retina (the screen). Move the magnifying glass to form a sharp image of the letter.

5. You have created an emmetropic eye, that is, an eye without refractive defects.

6. To simulate an eye with myopia, the retina will be removed from its initial position. In other words, the image will be formed before the screen. You can check it with the help of an extra white cardboard.

Figure 4.4.1 Materials.


Figure 4.4.2 Image formation in one eye.


• Flashlight• A letter printed on

acetate paper, which will leave an image

• DIN A4 black-and-white cardboard

• Plasticine (optional)

OBJECTIVES:Objective 1: Understand why there are myopic eyes and

hypermetropic eyes.Objective 2: Explain how these refractive defects are corrected.

MATERIALS• A fish tank with water• A magnifying glass (if you

don’t have one, you can place a converging lens inside a cardboard roll)

• A converging lens and a divergent lens

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Experiment: Why Do People Wear Eyeglasses?


4.4

7. To simulate a farsighted eye, we will follow the same procedure but this time bring the retina closer to the ini-tial position. The image will form behind the retina. As in the previous case, you can check it using another white cardboard.

8. Once you have created a shortsighted and farsighted eye, how do we make these eyes emmetropic? To correct shortsightedness and farsightedness, that is, to make the image formed in front of or behind the retina form in the retina, we place a divergent (negative) or convergent (positive) lens in front of the magnifying glass. Guess which one is which!

ExplanationThe refractive defect can be considered as a power error due to a mismatch between the equivalent power (cornea, lens power) and eye length. For example, if the equivalent power is too high for a certain eye length, the image is formed in front of the retina, and this results in a myopic refractive error. If the power is too low in relation to the length of the eye, the image is formed behind the retina and results in a hypermetropic refractive error. To correct a myopic eye, a divergent (negative) lens is interposed, instead a hyper-metropic or farsighted eye is corrected with a convergent (positive) lens (Figure 4.4.4).

Figure 4.4.3 Compensation of a simulated farsight-edness by approaching the beaker.


Figure 4.4.4 Simulation using ray tracing of the different ametropies.


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Experiment: Why Do People Wear Eyeglasses?4.4

Let’s see what you have learned• What is an emmetropic eye? What is the difference between a myopic eye and a hyper-

metropic eye?• If a person’s vision is corrected with divergent or negative glasses or contact lenses,

what refractive defect do they have?

Related experimentsExperiment 4.1 How does the human eye work? And that of other animals?

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Experiment

Do We See with Both Eyes?

30 min (+)Binocular vision, simultaneous vision, fusion, stereopsis, distance perception

4.5

If we have two eyes, it is not simply nature’s whim. Having binocular vision (Figure 4.10) gives us an in-depth vision, but to reach this, it is necessary to fulfil a series of previous degrees: simultaneous vision (first grade), fusion (second grade) and finally stereopsis,

that is, insight into depth (third grade). When a person does not have deep vision (it hap-pens to 8% of the population) and, for example, is not able to watch 3D movies, it may be because they fail any of the three degrees of binocular vision, but how can you know if you have binocular vision?

ProcedureSimultaneous vision and fusion 1. Take the sheet of paper and roll it up. 2. Place it in front of your right eye (it will be easier on the left if you are left-handed)

and look through it as if it were a spyglass. You must keep the other eye open. 3. Tilt the tube slightly towards the nose. 4. In the middle of the tube place your open hand, with the palm facing you. 5. Move it along the sheet of paper. What happens? Figure 4.5.1.

Figure 4.5.1 Position and result of the experiment when you have binocular vision: a hand with a hole.


OBJECTIVES:Objective 1: Experience the first degree that gives rise to binocu-

lar vision: simultaneous vision.Objective 2: Experience the third degree that gives rise to binocu-

lar vision: stereopsis.

MATERIALS• A sheet of paper• Two pieces of cardboard• Template for glasses• Red and blue tissue paper• A shoe box with lid• Scissors• Sticky tape or masking tape

• 2 drinking straws• Two coins or cardboard circles

of different sizes• Sheet of paper or cardboard of

a color that contrasts with the inside of the shoe box

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Experiment: Do We See with Both Eyes?4.5

Stereopsis (depth perception) 1. On a piece of cardboard, draw the

shape of spectacles twice. 2. Cut out both templates, including the

spaces where you will look through (we will call these windows).

3. Cut out two pieces of tissue paper, one red and one blue, larger than the win-dows already cut out.

4. Glue the pieces of tissue paper around the window’s edge. Try to stretch them out as far as possible.

5. Paste the second shape of cardboard you have cut out, making them both coincide and leaving the tissue paper squares in the centre.

6. Fold the cardboard to shape the pins. 7. Search online for “anaglyph video” and watch it with your glasses on. What differ-

ence does it make if you take them off (Figure 4.5.2)?

Correct distance? 1. Take the shoebox and remove the lid. 2. Cut an 8 x 13-cm window on one of the short sides of the box and make a hole in the

center of the cropped rectangle. The hole should be 0.5 cm in diameter, so you can see through it with one eye.

3. Cover the window you made in the box with the rectangle. To do this, put sticky tape at the bottom of the rectangle and paste it to the bottom of the window. The window must be covered by the rectangle.

4. On the opposite side of the box (in front of where the window is), make a hole for each straw. Make them the same height, with a little separation between them so the straws are held in place.

Figure 4.5.2 Viewing an anaglyph through the red-blue glasses.

Photography: IOS

Figure 4.5.3 Description of the assembly (left) and experimental assembly (right).

Sources: in house.

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Experiment: Do We See with Both Eyes?


4.5

5. Cut out the cardboard to the same height as the box. The width of the cardboard must be greater than that of the box, in order to make some flaps. Make two holes in the cardboard, at the same height and distance as the ones you made in the shoe box. Each straw must pass through these holes.

6. Glue the cardboard to the box. To do this, place the flaps where the holes of the straws are, so that the flaps are not visible through the box window.

7. Insert the straws through the holes. As there is also a hole in the cardboard, the straws will remain straight and aligned.

8. Use the two coins of different sizes to cut two circles of different sizes. The color of the circles must contrast with the color inside the box. Paste each circle to the end of a straw and...let’s start!

9. Place the straws at different distances from the window. Look through the hole in the window and try to move one of the straws until you think the two circles are the same size. When you are sure, take off the window and check it with both eyes.

ExplanationIn the first experiment, we are forcing each eye to see a different thing, with one the hole and with the other the hand. If there is simultaneous vision, you will see both images. If your brain also has the ability to merge, it will appear as if your hand has a hole (both images overlap). The position of the hole on the hand depends on the distance at which you have your hand with respect to yourself.

In the second experiment, your eye is again receiving two different images (anaglyphs), one bluish and one reddish, but from the same scene. Also, if you look at the scene, the images are laterally shifted with regard to each other. As the images are different, your brain can merge them and give them depth. The more you move between them, the more different they will appear, and the brain will give them greater depth. In the third experi-ment, it is seen that, when looking only with one eye, depth perception is affected. It is difficult to know which circle is farther away and how much. When checking with both eyes, we see two different images (one for each eye), and that provides enough information for our brain to analyze and discriminate the size, shape and distance of objects.

TricksOn the internet, there are web pages to generate anaglyphs from your own images. You can try it with a funny picture of yourself!

Let’s see what you have learnedDo you think we can differentiate depth with one eye?

Related experimentsExperiment 4.1 How does the human eye work? And that of other animals?

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Experiment

Retinal persistence, cinema45 min (+)

4.6

How Do We See Movement?

Due to the extraordinary characteristics of vision, optical illusion games can be cre-ated by using the underlying physical principle. As early as 1824, John Ayrton Paris was aware of this and invented the thaumotrope, an apparatus with which

he intended to demonstrate retinal persistence but which eventually became a popular game in Victorian England. The same happened with the zoetrope, created in 1834 by William George Horner, after which the same concept is hidden and which, in this case, was the precursor of the begin-nings of cinema. You can also build both devices and see for yourself what John and William already knew.

ProcedureBuilding a thaumotrope 1. Take a sheet of paper and fold it in

half. 2. Draw a fish on one side and a fish

tank on the other. Both drawings must face the same way. Check that the fish is smaller than the fish tank.

3. Paste the sheet, placing the straw between both sides.

4. Spin the straw around quickly and look at the paper. The fish is inside the fish tank!

Building a zoetrope 1. Glue the two ends of the cardboard. 2. Glue the cardboard rectangle on the

other side of the cardboard, so that a cylinder (drum) is formed. With the triangles on one of the edges, it will be easier to give it the circular shape. So that eyelashes are not visible, you can cover it with the other circle of black cardboard.

Figure 4.6.1 Thaumotrope on both sides.


Figure 4.6.2 Template to make the zoetrope.

Source: in house.

• Stiff cardboard• Black cardboard• Plastic beaker• Drawing pins

OBJECTIVES:Objective 1: Build a thaumotrope.Objective 2: Build a zoetrope.

MATERIALS• Sheet of paper• Drinking straw• Felt-tipped pens• Glue

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Experiment: How Do We See Movement?


4.6

3. To rotate the drum, take a plastic cup, place it face down and glue a piece of cardboard the size of a coin. Place the drum on top and hold it in place with a drawing pin.

4. On a piece of white cardboard measur-ing 50 cm wide and 5.5 cm high, draw a series of sequential drawings, e.g., an animal running.

5. Paste the ends of the cardboard and insert them into the drum.

6. Spin the drum and look through the windows. You will see how it looks like you see an animal running!

ExplanationBehind both experiments is the same principle: an image projected on the retina remains in it a very small time, 0.1 seconds, before disappearing completely. If after this image another one is presented, then the brain will interpret them as being together. In the first case, the fish seemed to be inside the fishbowl, as the two images overlapped, and that is why we can also build an illusion of movement through static drawings using the zoetrope.

TricksFor the first part, you can use infinite pairs of images: a bird and a cage, a fried egg and a pan, a window and a face, a bouquet and a vase, the two sides of a coin...it’s up to your imagination! The same happens in the second part: a balloon that flies, a person running, someone putting a basket or a trapeze artist. You set the limits! On the internet, you have a lot of options available that you can print and use.

Let’s see what you have learnedIn the second part, why does the strip of images get into a drum, and why do you have to look through the windows?

Figure 4.6.3 Black cardboard measurements and of the parts to be assembled for construction of the zoetrope and clippings.


Figure 4.6.4 Final result of the experimental setup and the moving image strip.


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Experiment

Optical illusions30 min (+)

You Won’t Believe What You See!

Optical illusions are quite simple phenomena that deceive our brain. For example, a circle with a black

half and the other half white and black lines drawn is a very simple structure that, turning it quickly, will make you see dif-ferent colors. This phenomenon is called “Benham’s disk”, and there are people who see red, green, blue, brown...And you, what colors do you see?

Surely you have heard that vertical striped shirts flatter the figure because they make you look taller and thinner, while horizontal striped shirts have just the opposite effect, but is this true? With only one piece of paper and a pen, you can check this out. It is as easy as that!

ProcedureBenham’s disk 1. Cut out a circle in the cardboard of the

same size as the disk you are going to use. You can use the template to draw Benham’s disk or print it online. Mark the center of the cardboard circle with a cross.

2. Paste the disc template on the cardboard.

3. Use the stick to punch the cross. 4. Enter the small wooden cylinder. 5. Turn your disk with the template up

and pay attention to the lines. What can you see?

Figure 4.7.1 Benham’s disk.

Source: Selket, Wikimedia Commons.

Figure 4.7.2 Benham disk assembly.

Source: in house.

• Small wooden cylinder• Benham’s disk template• Sheet of paper (best graph)• Ballpoint pen

OBJECTIVESObjective 1: Understand the psychophysical effect behind the

optical illusion of Benham’s disk and its variations.Objective 2: Understand the illusion of Helmholtz, in its usual

format with squares of vertical lines and in its three-dimensional human form.

MATERIALS• Cardboard• Scissors• Glue• Ruler• Toothpick

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Experiment: You Won’t Believe What You See!


4.7

Helmholtz square theory 1. Draw on a piece of paper the same

number of vertical and horizontal lines, equidistant from each other. You will have two squares of identi-cal areas.

2. Which group of lines do you think is narrower and longer?

3. Now have someone dress in striped vertical and horizontal clothes. Which do you think streamlines you the most?

Different optical illusions 1. Look at Figure 4.7.4; the circles seem to move! 2. If you now look at Figure 4.7.5, can you see the black dots in the white circles? Do

they disappear when you look at them?

ExplanationIn Benham’s disk, there are only two colors, black and white. In order to see the color white, the three types of cones must respond equally, however, in each person they have different response times. As in this template, both colors, black and white, are alternating rapidly, and each person sees different shades, as the color receptors respond. If we vary the speed of rotation of the disk, we will see how the saturation of the colors we perceive varies and, furthermore, if we vary the direction of rotation of the disc we will observe how the colors are reversed.

Helmholtz (1867) found that a square composed of horizontal lines seems to be too tall, and one composed of vertical lines seems too wide. This effect is observed in situa-tions of everyday life, such as an empty room that looks smaller than one that is furnished, and a wall covered with a paper pattern that looks larger than an evenly painted one color. The exact explanation is still unknown, but stud-ies have been conducted, the last led by the expert perception psychologist Peter Thomson, in which this theory was con-firmed. In human figures, this effect of thinness or width is observed especially in the hips; the vertically striped pattern makes the hips appear wider, the outline of the silhouette being the same, so it can be denied that the vertical stripes have a slimming effect.

Figure 4.7.4 shows an optical illusion known as rotating snakes, created by

Figure 4.7.3 Helmholtz square theory.

Source: in house.

Figure 4.7.4 Optical illusion of rotating snakes.

Source: Wikimedia Commons.

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Experiment: You Won’t Believe What You See!4.7

scientist Akiyoshi Kitaoka. These circles were created following patterns of colors and shadows similar to those we receive from a moving object. That is why it seems that they are in continuous movement. This type of illusion affects peripheral vision; if we stare at a single circle, the movement will stop.

Figure 4.7.5 shows the optical illusion of the scintillating grid, a variant of the Hermann grid. This optical illusion is due to the complex functioning of the ganglion cells that form the retina and to the lateral inhibition process. This, in short, means that the intensity of the specific point is the result of the sum of different cells, so the distance between the grid lines causes our cells to be confused and omit the intersec-tion between squares.

Tricks• Not everyone sees the same thing when the disc spins; try it with your friends.• Search for optical illusions on the internet and discover how to cheat your brain in a

fascinating way!

Let’s see what you have learned• Why is it necessary for Benham’s disk to spin?• If you want to recommend someone a type of print to streamline their figure, what

would you recommend? Vertical or horizontal stripes?

Related experimentsExperiment 4.6 How do we see movement?

Figure 4.7.5 The twinkling grid.

Source: Wikimedia Commons.

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Chapter 4 The human eye: a biological camera