# Rectilinear propagation of light

Do you know why the eclipse happens? There are two types of eclipse: one is the solar eclipse and the other is the lunar eclipse. In a solar eclipse the moon comes in between the earth and the sun and we are not able to see the sun. In a lunar eclipse the earth comes in between the moon and the sun and the light coming from the sun does not reach the moon properly and it looks as if it disappears. These phenomena are possible due to the rectilinear properties of light. Lets learn more about this behaviour of light in detail.

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## Rectilinear propagation of light

The rectilinear propagation of light is a phenomenon which tells that light travels in a straight line. It can change its direction only when either it is reflected from a surface or change its medium of propagation, i.e. refraction. This concept of rectilinear propagation of light is used to understand the various optical phenomena.

There are some experiments such as the cardboard experiment and rubber tube experiment which describe the rectilinear propagation of light. Let's discuss them one by one.

## Cardboard Experiment

This experiment proves the rectilinear propagation of light. It is a simple experiment and does not need any specific instrument. For the cardboard experiment you need a simple light source such as a candle or bulb or a lamp and some cardboard along with a sharp pin which is needed for making the holes in the cardboards.

For doing this experiment just place the light source on a plane surface. Now using the pin make holes in the cardboards and place three or four cardboards parallely in different planes at some distances so that these holes will remain in a line. Light the source of light and place it along the holes. See this light source from the hole of the last cardboard and note the observation.

Now displace one of the cardboards (let's say, the middle one) a little downward as shown in the figure below. Now try to see the source from the hole of the last cardboard and note the observation.

In the first case when the holes are aligned, the light is reaching your eyes and you can see the light source clearly. But when the middle cardboard is displaced downward the light is not able to reach you and thus, the source is not visible through the holes. Hence, it proves the rectilinear propagation of light.

## Rubber Tube Experiment

This is a simple experiment which proves the rectilinear behaviour of light and can be done at home. For doing this experiment you need a flexible rubber tube and a light source such as a candle or a bulb or a lamp.

The procedure of doing this experiment is very simple. Just light the light source and place it on the table. Now take the rubber tube and place it in front of the source such that one end should be towards the light source and other end should be towards you. The tube should not have any bend in it. Now you can see the candle through the rubber tube and note the observation. Now, you bend this rubber tube at some point in the middle and try to see through the tube and note the observation.

In the first observation, you will find that you can see the source of light clearly. Which proves the rectilinear propagation of light. In the second observation when the tube is bent, you are not able to see the light source because the bend in the tube blocks the light reaching your eyes. Hence, this experiment proves the rectilinear propagation of light behaviour.

## Rectilinear propagation of light in daily life

There are few daily life examples where the rectilinear behaviour of propagation of light can be observed.

1. Eclipse : Solar eclipse and lunar eclipse are the examples of rectilinear propagation of light. In the solar eclipse, the moon comes in between the earth and sun. The light coming from the sun in a straight line gets obstructed by the moon and it seems that the sun disappeared partially. In the lunar eclipse, the earth comes in between the sun and the moon and obstructs the straight rays of light. Thus, the moon doesn’t get any light from the sun and seems to have disappeared.

Solar Eclipse

Lunar Eclipse

1. Formation of the shadow : It is a good example of rectilinear behaviour of the propagation of light. When a light falls on an object, it stops the light rays and a shadow is observed behind the object.
2. The light from the torch and headlight of a vehicle travels in a straight line.
3. Light from the projector travels in the straight line and reaches the screen.
4. Light from a laser travels in a string line.

## Properties of light

Here are some properties of light.

1. Light can be considered a wave of particles.
2. Like all waves it also carries energy.
3. Light propagates in a straight line, this property is known as rectilinear propagation of light.
4. Light does not involve the medium of propagation; it can travel in the vacuum also.
5. Light travels with the speed of 3 x 108 m/s in the vacuum or in air.
6. The propagation of light does not affect the medium but when light changes the medium from rarer to denser, its velocity gets reduced.
7. The portion of the electromagnetic radiation spectra which falls in the visible spectrum is known as visible light.
8. The visible light can be seen with human eyes.
9. The wavelength of visible light is from 380 to 700 nm.

## Practice Problems

Q1. Define the optic?

A. Optic is the branch of physics which deals with the behaviour and properties of light.

Q2. What is the reflection of light?

A. Reflection of light is defined as the change in the direction of the path of the light. It is done by a reflecting surface such as a mirror. We can also do the reflection by glass, water, polished metallic surfaces etc.

Q3. What is the refraction of light?

A. When a light wave travels from one medium to another medium, it changes its path. The light ray bends at an angle. The angle depends upon the mediums from which the light is travelling and where it enters. The deviation in the path of the light ray is only when the light ray falls on the interface at an angle which is not perpendicular.

Q4. Define the interference of light.

A. When two light rays coming from monochromatic sources are superimposed then there is high intensity of light at some points and there is low intensity of light at some points. This phenomena is called the interference of light. Where the crests (or troughs) of the two waves superimpose, they create constructive interference. Where the crest of one wave is superimposed with the trough of the other wave then destructive interference occurs.

## FAQs

Q1. Who was the first to observe the rectilinear propagation of light?
A.
Pierre de Fermat discovered the rectilinear propagation of light.

A.
Shadow is a dark area where the light is not able to reach due to some opaque object in its path.

Q3. What are the laws of reflection?
A.
The laws of reflection are:

The angle of reflection will be equal to the angle of incident.
The incident ray, the reflected ray and the normal to the surface will always be on the same plane.

Q4. Which medium has the maximum speed of light?
A.
Speed of light is maximum in vacuum. It is close to 3 x 108 ms-1

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