PHYSIOLOGICAL OPTICS 6TH LECTURE

Dr. Mohammad Shehadeh

REFRACTION OF LIGHT

Refraction: is the change in direction of light when it passes from one transparent medium into another of different optical density.

The incident ray, the refracted ray and the normal all lie in the same plane.

The velocity of light varies according to the density of the medium through which it travels.

The more dense the medium the slower the light passes through it.

When a beam of light strikes the interface separating a less dense medium from a denser one obliquely, the edge of the beam which arrives first, A, is retarded on entering the denser medium.

The opposite side of the beam, B, is meanwhile continuing at its original velocity.

The beam is thus deviated as indicated in Fig being bent towards the normal as it enters the denser medium.

ABSOLUTE REFRACTIVE INDEX, N, OF THE MEDIUM

It is a comparison of the velocity of light in a vacuum and in another medium (optical density of that medium)

As the optical density of air as a medium is negligible under normal conditions

EXAMPLES OF REFRACTIVE INDEX ARE:

Air = 1

Water (incl. Aqueous) = 1.33  Cornea = 1.37  Crystalline lens = 1.386–1.406  Crown glass = 1.52  Flint glass = 1.6  Diamond = 2.5

On entering an optically dense medium from a less dense medium, light is deviated towards the normal.

The incident ray makes an angle, i, the angle of incidence, with the normal.

The angle between the refracted ray and the normal is called the angle of refraction, r.

SNELL'S LAW

Snell's law states that the incident ray, refracted ray and the normal all lie in the same plane

angles of incidence, i, and refraction, r, are related to the refractive index, n, of the media concerned by the equation

where the first medium is a vacuum, n is the absolute refractive index,

and in air n is the refractive index. on passing from medium1 into

medium2, the index of refraction is given by

Light passing obliquely through a plate of glass is deviated laterally and the emerging ray is parallel to the incident ray.

Thus the direction of the light is unchanged but it is laterally displaced

some reflection also occurs at every interface

a lens or window with a refractive index of 1.5 in air reflects 4% of light from the anterior surface and transmits the remaining 96% to the posterior surface;

a further 4% of this is reflected so that the lens transmits only 92.16% of normally incident light

a sheet of glass as an image-splitter, e.g. the teaching mirror of the indirect ophthalmoscope.

Most of the light is refracted across the glass to the examiner's eye.

However, a small proportion is reflected at the anterior surface of the glass and enables an observer to see the same view as the examiner.

REFRACTION OF LIGHT AT A CURVED INTERFACE

Light passing across a curved interface between two media of different refractive indices obeys Snell's law.

A convex spherical curved surface causes parallel light to :

1. converge to a focus if n2 is greater than n1,

2. diverge as from a point focus if n2 is less than n1

The refracting power or vergence power of such a surface is given by the formula:

• r: is the radius of curvature of the surface in metres • The surface power is measured in dioptres • Surface power is positive for converging surfaces and negative in sign for diverging surfaces

Objects situated in an optically dense medium appear displaced when viewed from a less dense medium.

This is due to refraction of the emerging rays which now appear to come from a point I, the virtual image of object O .

Objects in water seem less deep than they really are.

RAYS EMERGING FROM A DENSER MEDIUM TO A RARER MEDIUM SUFFER A VARIETY OF FATES, Ray A strikes at

90° to the interface and is undeviated

Ray B emerges after refraction.

Ray C, runs parallel wi h the interface (the critical angle)

TOTAL INTERNAL REFLECTION.

Rays striking more obliquely than the critical angle still fail to emerge from the denser medium and are reflected back into it as from a mirror.

The critical angle is determined by the refractive indices of the media involved and can be calculated using Snell's law.

The critical angle for the tear film/air interface is 48.5°, and for a crown glass/air interface the critical angle is 41°.

Total internal reflection is used in optical instruments:

Fibre optic cables1. surgical intraocular light source and 2. the transmission of laser light from

the laser tube to the delivery system of the laser slit lamp.

Total internal reflection also occurs at

surfaces within the eye the cornea:air interface, and prevents

visualisation of parts of the eye, e.g. the angle of the anterior chamber and peripheral retina.

The problem is overcome by applying a contact lens made of material with a higher refractive index than the eye and filling the space between eye and lens with saline, thus destroying the cornea/air refracting surface and allowing visualisation of the anterior chamber angle (gonioscopy) and peripheral retina (three-mirror).

DISPERSION OF LIGHT In fact, the refractive index of any medium

differs slightly for light of different wavelengths.

Light of shorter wavelength is deviated more than light of longer wavelength, e.g. blue light is deviated more than red.

The refractive index of a material is normally taken to mean that for the yellow sodium flame.

 The angle formed between the red and blue light around the yellow indicates the dispersive power of the medium

This is not related to the refractive index of the material.

THE RAINBOW:

Total Internal Reflection and Dispersion When sunlight enters a raindrop it is dispersed

into its constituent spectral colours Under certain circumstances, the angle of

incidence is such that total internal reflection then occurs within the drop.

The dispersed light finally emerges, each wavelength or colour making a different angle with the horizon.

To see the rainbow, the observer must look away from the sun.

The observer receives only a narrow pencil of rays from each drop, i.e. only one colour.

The whole rainbow is the result of rays received from a bank of drops at increasing angle to the observer's eye

Violet, the colour making the smallest angle to the horizon, is received from the lower drops while red, making the greatest angle with the horizon, is received from the highest drops

Thus the red is on the outside of the primary rainbow.

The secondary rainbow is formed by rays that have twice undergone total internal reflection within the raindrops, and the colours are seen in reverse order: violet is on the outside of the bow.