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# Dispersion

Splitting of white light into its seven constituent colours (wavelength), on passing through a transparent medium is known as dispersion of light.

Dispersion

Activity 7

Place a prism on a table and keep a white screen near it. Now, with the help of a torch, allow white light to pass through the prism. What do you see? You can observe that white light splits into seven colored light rays namely, violet, indigo, blue, green, yellow, orange and red (VIBGYOR) on the screen. Now, place another prism in its inverted position, between the first prism and the screen. Now, what do you observe on the screen? You can observe that white light is coming out of the second prism.

Aim : To understand about dispersion of light.

Materials required : Prism - 2, white screen, torch light.

Procedure :

(i) Place a prism on a table and keep a white screen near it.

(ii) Now with the help of a torch, allow white light to pass through the prism. What do you see?

(Hi) Now, place another prism in its inverted position, between the first prism and the screen. What do you observe on the screen?

Observation I : We can observe that white light splits into seven coloured light rays namely, violet, indigo, blue, green, yellow, orange and red (VIBGYOR) on the screen.

Observation II : (After placing second prism)

(i) We can observe that white light is coming out of the second prism.

(ii) The first prism splits the white light into seven coloured light rays and the second prism recombines them into white light again.

Inference : It is clear that white light consists of seven colours.

Conclusion : Splitting of white light into its seven constituent colour, on passing through a transparent medium is known as dispersion of light.

In this activity, you can see that the first prism splits the white light into seven coloured light rays and the second prism recombines them into white light, again. Thus, it is clear that white light consists of seven colours. You can also recall Newton’s disc experiment, which you studied in standard VII.

Splitting of white light into its seven constituent colours (wavelength), on passing through a transparent medium is known as dispersion of light.

Why does dispersion occur? It is because, light of different colours present in white light have different wavelength and they travel at different speeds in a medium. You know that refraction of a light ray in a medium depends on its speed. As each coloured light has a different speed, the constituent coloured lights are refracted at different extents, inside the prism. Moreover, refraction of a light ray is inversely proportional to its wavelength.

Thus, the red coloured light, which has a large wavelength, is deviated less while the violet coloured light, which has a short wavelength, is deviated more.

The formation of rainbow is an example of dispersion of white light. This can be seen on the opposite side of the Sun. After rainfalllarge number of droplets still remain suspended in the air. When white light passes through them, it is split into seven colours. Dispersion of white light from a large number of droplets eventually forms a rainbow.

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