Refraction of light through a glass prism

Refraction is the bending of light when it goes from one medium to another so, when a ray of light passes through a glass prism, refraction of light occurs both, when it enters the prism as well as when it leaves the prism. Since the refracting surfaces are not parallel, therefore, the emergent ray and incident ray are not parallel to one another. In this case the ray of light is deviated on passing through the prism.

A glass prism is a transparent object having two triangular ends and three rectangular sides. The refraction of light in glass prism is different from a glass slab. This is because in glass prism, the incident ray of light is not parallel to emergent ray of light.

When a ray of light enters the glass prism it gets deviated two times. First when it enters the glass prism and second when it comes out of the prism. This is because the refracting surfaces of the prism are not parallel to each other. Also, when the ray of light passes through the prism it bends towards its base.


Dispersion of light

In 1665, Isaac Newton discovered that white light consists of seven colours. He found that if a beam of white light is passed through glass prism then it will split in to seven colours. These colours are red, orange, yellow, green, blue, indigo and violet (VIBGYOR).

Spectrum of white light: The band of seven colours formed when a beam of white light is passes through a glass prism is called spectrum of white light.

Dispersion of light: The splitting of white light into seven colours on passing through a transparent medium is called dispersion of light.

The dispersion of white light happens because the angle of refraction of lights of different colours is different while passing through the transparent medium. For example, red colour deviates least and is formed at the upper part of the spectrum and violet colour is deviated maximum and is formed at the bottom of the spectrum.


Recombination of spectrum of colours

A spectrum of seven colours of lights can be recombined to form back white light. This can be done by placing two glass prisms side by side. But, place the second glass prism in an inverted position. When white light passes through first prism it disperses the light into seven colours and when this beam of light enters the second prism which is placed in an inverted position, a white light is obtained when it comes out of the second prism.

The seven coloured lights recombine to form white light due to second glass prism which is placed in a reversed position.


The rainbow

Rainbow is formed when it is raining at the time of sunshine. When the white sunlight falls on the raindrops and leaves them, then the white light is refracted and an arc of seven colours is formed in the sky. In this situation, tiny raindrops act as glass prism splitting the white sunlight.


Atmospheric refraction

When the refraction of light takes place due to earth’s atmosphere it is called atmospheric refraction. So, when light ray enter the atmosphere there is air and every air layer has different temperature. These air layers have different optical densities. Cooler air layer is an optically denser medium for light rays whereas warmer air layer is optically rarer medium for light rays.

The following are the examples of atmospheric refraction of light.

1) Twinkling of stars

Stars twinkle at night because their light is refracted in the atmosphere. When the light of star enters the earth’s atmosphere it undergoes refraction due to different optical densities of the air. Therefore, stars appear bright at one moment and dim in another.

2) Stars appear higher than they are

The light from stars is refracted as it comes down into earth’s atmosphere. The air higher up in the sky is rarer and near the earth’s surface is denser. As the star light falls down the dense air bends it more and thus stars appear higher than they actually are.


3) Advance sunrise and delayed sunset

It is due to refraction of light that we are able to see the sun two minutes before sunrise and two minutes after actual sunset. At the time of sunrise the sunlight is coming from less dense air to more dense air. In this case the sunlight is refracted downwards and because of this sun appears to be raised above the horizon than it actually is.


Scattering of light

Throwing light in various random directions on various types of suspended particles is called scattering of light.

Tyndall effect

When light is scattered due to particles in its path, it is called Tyndall effect. The way a beam of sunlight becomes visible when it passes through dust particles in a room, when sunlight passes through a canopy of dense forest etc., are examples of Tyndall effect.

In 1859, Tyndall discovered that when white light is passed through clear liquid having small suspended particles, then the blue colour of white light has shorter wavelength and is scattered more than the red colour that has longer wavelength.

The colour of scattered light depends on the size of particles

  • It is due to the scattered large dust particles and water droplets in the atmosphere that when white sunlight falls on them it is reflected in such a way that the scattered light also appears white. Dust particles and water droplets in the atmosphere are larger than the wavelength range of the visible light.
  • The extremely small air molecules in the atmosphere scatter mainly blue light when white sunlight falls on them. This is because blue colour has lower wavelength and is much more by the air molecules.

Why the sky is blue?

When white sunlight falls in the atmosphere, lights with longer wavelength are not scattered by the air molecules. It is only the blue light which has shorter wavelength that is scattered most by the air molecules in the atmosphere. This is why the sky looks blue.

Why the Sun appears red at sunrise and sunset?

At the time of sun rise and sunset all the blue coloured light is scattered out and is away from our sight. So the light reaching us mainly at the time of sunrise and sunset is red which has longer wavelength.

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