Light Travels In a Straight Line

Light travels in a straight line can be observed by keeping an object in

the path of light. In an atmosphere which is bit dusty, we can see light

traveling in a straight line. Light emerging from the torch, train and

lamps always travel in a straight line. Let us study in detail how does

light travel in a straight line.

Light Travels Along a Straight Line

Life without light would have been pretty dull. Light travels at a speed

of 186,000 miles per second. You must have observed that in your

house that whenever a beam of light enters a dark room through a tiny

hole in the window, the lightwave always travels in a straight line.

Let us carry out a small activity to show that lightwave travels along a

straight line. Take three CD’s and align them together. Align them in

such a way that all the CD’s line in a straight line. Now take a candle

and place it at the other end. Do make sure that the tip of the candle

and the holes of the CD’s all lie in the straight line. Ensure that the

height of the CD’s and the tip of the candle are same. Observe the

flame of the candle. We are able to see the flame of the candle because

the light wave travels through the holes and reaches our eye.

Now if suppose we displace the center of the CD’s we observe that we

are not able to see the flame of the candle. Why does that happen?

This is because the light gets blocked. If the light could have the

ability to take a curve and travel, we could have seen the lightwave.

But since light travels in a straight line, we were unable to see the

flame of the candle when the CD is displaced. This proves that light

travels along a straight line.

(Source: Wikipedia)

In the above picture, we can clearly see that light coming through the

holes in the window travel along a straight line.

Questions For You

Q1. The phenomenon in which the moon’s shadow falls on earth, or

the earth casts its shadow on the moon, is known as

A. Shadow

B. Lateral deviation

C. Eclipse

D. Tides

Answer: C. The phenomenon in which the moon’s shadow falls on

earth or the earth casts its shadow on the moon is known as an eclipse.

During a solar eclipse, moon’s shadow falls on the earth. During a

lunar eclipse, earth’s shadow falls on the moon.

Q2. Two examples of non-luminous objects are

A. Stars and Moon

B. Burning candle, glowing bulb

C. The moon, a spoon

D. Stars, a spoon

Answer: C. Non-luminous objects are those that do not emit light. The

moon and the spoon do not emit light. So these two are good examples

of non-luminous objects.

Q3. We can see the objects only when

A. Reflected light from the object reaches our eye.

B. The objects absorb all the light.

C. When the objects allow all the light to pass through them.

D. None of these.

Answer: A. Objects can only be seen when light falls on the object

and are reflected back to our eyes.

Mirrors

When you look in the mirror have you noticed something interesting

about you and the image of you in the mirror? Let us carry out a small

activity. Stand in front of the mirror and move your right hand. Now

lift your left hand. Did you notice that in the mirror the right appears

left and vice versa? To understand this let us talk about Mirrors.

Mirrors

A smooth and highly polished reflecting surface are is a mirror. Most

commonly used are plane mirrors.

Lateral Inversions in Plane Mirrors

The word ‘AMBULANCE’ is opposite because when the driver of the

vehicle ahead of the ambulance looks in her/his mirror, the person can

read it as ‘AMBULANCE’ and give way to it.

Spherical Mirrors

Suppose you are sitting at the dining table and you don’t like the food,

you start playing with the spoon. You look yourself in the spoon and

you notice that you look pretty funny. The moment you get the spoon

closer you get a magnified image and when taken far, you see an

inverted image. Do you know what’s really happening? To understand

what is happening lets us talk about the special class of mirrors known

as spherical mirrors.

Let us first understand the terms of spherical mirrors.

● Radius of Curvature (c): It is the distance between Pole and the

Center of curvature.

● Center of Curvature (r): The Center of Curvature of a spherical

mirror is the point in the center of the mirror which passes

through the curve of the mirror and has the same tangent and

curvature at that point.

● Aperture: It is a point from which the reflection of light

actually happens.

● Pole (p): Pole is the midpoint of a mirror. It’s twice the focus.

● Focus: It is any point, where light rays parallel to the principal

axis, will converge after reflecting from the mirror.

● Principal axis: An imaginary line passing through the optical

center and the center of curvature of the spherical mirror.

● Focal Length: It is on the axis of a mirror where rays of light

are parallel to the axis converge after reflection or refraction.

Spherical mirrors are of two types: ● Convex Mirror

● Concave Mirror

Concave Mirror

We are familiar that the spherical mirrors are not plane, they are

curved in one particular direction. They are curved inward. A concave

mirror is also known as the converging mirror as in these type of

mirrors light rays converge at a point after they strike and are getting

reflecting back from the reflecting surface of the mirror.

A concave mirror produces real and inverted images except when the

object is placed very near to the mirror that pole (p) and the focus (f)

where the image produced is virtual and erect. The concave mirror is

used in shaving mirrors to see a large image of the face. It is also

useful in vehicle headlights and torches. Dentists also use a concave

mirror to get the bigger image of the teeth.

Convex mirror

The convex mirror has a reflective surface that curves outward. These

mirrors are “always” form virtual, erect and diminished regardless of

the distance between the object and mirror.

When parallel rays of light strike the mirror, they are reflected in a

way wherein they spread out or diverge. For this reason, a convex

mirror is also a diverging mirror too. If these reflected rays are

extended behind the mirror by dotted lines, they meet at a point. This

point is the focus of the convex mirror. The concave mirror is used in

the vehicle so that the driver is aware of the vehicle coming from

behind. They are also used in street light reflectors.

Question For You

Q. The following is used in the rear-view mirrors of vehicles:

A. Plane Mirror

B. Convex Mirror

C. Convex Lens

D. Any of these

Answer: B. The Convex Mirror is used in the rear-view mirrors of

vehicles as it forms upright/erect images and has a wider field of view.

Reflection of Light

We ‘see’ a lot of things around us on a daily basis. But how are we

able to do so? You might answer saying it is because of our eyes. Isn’t

it? And you aren’t wrong. But is it just the eyes? What about the light

bounced back from the things around us and enabling us to see things

better? What is this phenomenon? It’s because of ‘Reflection of

Light’. Let us study more about it below.

What is Reflection of Light?

When a ray of light falls on any object (polished, smooth, shiny

object), light from that object bounces back those rays of light to our

eyes and this is known as “Reflection” or “Reflection of Light”.

This phenomenon is what enables us to look at the world around us

based. Before, after and during reflection light travels in a straight

line. For example, twinkling of stars or light reflected by a mirror.

Laws of Reflection

In the diagram given above, the ray of light that approaches the mirror

is known as “Incident Ray”. The ray that leaves the mirror is known as

“Reflected Ray”.

At the point of incidence where the incident ray strikes the mirror, a

perpendicular line is drawn known as the “Normal”. This normal is

what divides the incident ray and the reflected ray equally and gives

us the “Angle of Incidence” (Qi) and “Angle of Reflection” (Qr).

Hence the above information gives us the “Laws of Reflection of

Light” which state that :

a. The angle of incidence is equal to the angle of reflection.

b. The incident ray, the normal and the reflected ray, all lie in the same

plane.

Types of Reflection:

There are two types of reflection :

a. Specular/ Regular reflection

b. Diffused/ Irregular reflection

a. Specular/ Regular reflection:

Specular/Regular reflection is the mirror-like reflection of rays of

light. In such a type of reflection, the rays of light that are reflected

from a smooth and shiny object such as a mirror, are reflected at a

definitive angle and each incident ray which is reflected along with the

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reflected ray has the same angle to the normal as the incident ray.

Thus, this type of phenomena causes the formation of an image.

b. Diffused/ Irregular Reflection:

Diffused/Irregular reflection is a non-mirror-like reflection of light. In

such a type of reflection rays of light that hit an irregular object with a

rough surface, are reflected back and scatter in all directions. Here, the

incident ray which is reflected along with reflected ray doesn’t have

the same angle to the normal as the incident ray.

Thus, this type of reflection doesn’t form an image.

Image Formation by Plane Parallel Mirrors

As shown in the figure, infinite images are formed when an object

placed is placed between two parallel plane mirrors.

Consider an object placed in between the mirrors and we can actually

see the image, in this case, the number of images would range from 1

to X (X being a very large number depending on your position relative

to the mirror and object, and X not infinity).

Angle between mirrors = θ=O

Hence,

Number of images formed = 360/θ

So, in case of parallel mirrors, we get infinite images.

Solved Example For You

Q. Which of the following phenomenon has two angles that are equal

to each other?

a. Reflection

b. Refraction

c. Diffraction

d. None of the above

Sol: a. Reflection

According to Snell’s law of reflection, the angle of reflection with

normal is equal to the angle of incidence with that normal and all these

lies in the same plane.

Sunlight- White or Coloured

As we all know that Sun is the major source of light. But what is the

color of the sunlight? We see the rainbow in the sky consisting of

different colors that usually appears after the rains when the sunlight is

low. Also, the sky appears orange or red during sunset. So is sunlight

colorful? Let us learn this in detail and see what is the actual color of

sunlight.

Sunlight- White or Coloured

White light is called as white because it consists of seven colors. The

sunlight splits into seven colors namely violet, indigo, blue, green,

orange, and red. We usually call it as VIBGYOR. When we mix all

these colors we just get one light which is the WHITE light. Let us

carry out a small activity to show that sunlight has seven different

colors.

With the help of a narrow beam of light, a glass prism, and a white

wall it is possible to produce the band of seven colors using white

light. Keep this arrangement near the window. Place the glass prism in

such manner that the sunlight through the window falls on one side of

the prism and then on the white wall. You can see that the light

reflected on the wall has several colors. The prism splits the white

light into seven different colors. This splitting of white light into many

colors is called as a dispersion of light.

This shows that the sunlight consists of several colors. Sometimes in

the rainbow, you may not see all the seven colors. This is because of

the colors overlap each other. The degree of bending of the light’s

path depends on the angle that the incident beam of light makes with

the surface, and on the ratio between the refractive indices of the two

media (Snell’s law) and results in splitting all the seven colors present

in white light.

Spectrum

It is always not necessary that the source of light should always be

white to get a spectrum. The composite light which contains a range of

three to four colors also produces a band of three to four colors. A

glowing 40W tungsten filament bulb does not produce pure white light.

It is a source of composite light and will also produce a spectrum but

the spectrum may not be the same as the spectrum of white light. The

type of spectrum depends upon the nature of the source of light.

Dispersion of Light In Daily Life

Examples of dispersion in our daily life:

● After the rains, we see the rainbow in the sky which is due to

the dispersion of the sunlight.

● During rainy days when the roads are wet and you drive the car

or ride a bike sometimes the water spills on the road. When the

petrol mixes with the water we can see different colors.

● When sunlight passes through the prism the light splits into

different colors.

● Dispersion of colors in soap bubbles.

● Dispersion of colors on CDs.

Question For You

Q. Blue color of sea water is due to:

A. Interference of sunlight reflected from the water surface.

B. Scattering of sunlight by water molecules

C. Image of sky in water

D. Refraction of sunlight

Ans: A. The ocean looks blue because red, orange and yellow are absorbed

more strongly by the water than the blue.

Images Formed By Lenses

Have you ever noticed the watch repairer? How does he manage to

locate and fix such tiny particles of the watch? What do you think

does he uses to view such small parts? Yes, he uses an eyepiece that

has a lens that magnifies the parts of the watch. So let us now study

the types of lenses and also the images formed by lenses.

Images Formed by Lenses

A lens is a part of a transparent thick glass which is bounded by two

spherical surfaces. It is an optical device through which the rays of

light converge or diverge before transmitting.

Types of Images Formed by Lenses

Lenses are of two kinds: Convex Lenses and Concave Lenses.

Convex Lenses

A convex lens is thicker in the middle and thinner at the edges. A

convex lens is also known as a “biconvex lens” because of two

spherical surfaces bulging outwards.

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Convex lenses include lenses that are plano-convex (i.e. these lenses

are flat on one side and bulged outward on the other), and convex

meniscus (i.e. these lenses are curved inward on one side and on the

outer side it’s curved more strongly).

A concave lens is thicker at the edges and thinner in the middle. A

concave lens is also known as a “biconcave lens” because of two

spherical surfaces bulging inwards.

Concave lenses include lenses like plano-concave (i.e. these lenses are

flat on one side and curved inward on the other), and concave

meniscus (i.e. these lenses are curved inward on one side and on the

outer side it’s curved less strongly).

Image Formation by Concave Lens

In case of the concave lens, we always get erect images, diminished

images and virtual images.

Object location Image location Image nature Image size

Infinity At F2 Virtual and Erect Highly Diminished

Beyond infinity

and 0 Between F1 and

Optical centre Virtual and Erect Diminished

Here if the object is very far away, the images formed by lenses will

be all the more diminished.

Image Formation by Convex Lens

In case of a convex lens, if we bring the object close to the lens, the

size of the image keeps on increasing. As you bring the object more

close to the lens, we get the image all the more enlarged. So here we

can say that the images formed can be of a variety of types. We can

have diminished inverted image, small sizes inverted image, enlarged

inverted image, enlarged erect image. So in a concave lens, there is a

possibility of getting a real as well as an inverted image.

Object location Image location Image nature Image size

Infinity At F2 Real and Inverted Diminished

Beyond 2F1 Between 2F2 and

F2 Real and Inverted Diminished

Between 2F1 and

F1 Beyond 2F2 Real and Inverted Enlarged

At F1 At infinity Real and Inverted Enlarged

At 2 F1 At 2F2 Real and Inverted Same size

Between F1 and 0 On the same side

as object Virtual and Erect Enlarged

Images Formed in Different Lenses

Which lens would you use as a magnifying glass?

Looking at the image what do you think? What kind of image is

formed? Yes, an enlarged image is formed. We cannot get such image

using a concave lens because the concave lens always produces the

diminished image. So this is a convex lens.

Images through lenses as real, virtual, erect or magnified

The figure shows the virtual image formed by the convex lens and real

image formed by a concave lens.

Learn more about the Different type of Mirrors here.

Solved Examples for You

Question. How should people wearing spectacles work with a

microscope?

A. Should keep wearing their spectacles

B. They should never use the microscope

C. Should take off their spectacles

D. They should either keep wearing their spectacles or take off

their spectacles.

Solution: Option C. If operator using microscope usually wears

spectacles for working at their PC’ s, they often need to remove them

while looking through the microscope so they can align their eyes

correctly.

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