Scattering of lights of different colours


 
 
Concept Explanation
 

Scattering of Lights of Different Colours

The colours of light scattered by a suspended particle depends on the size of the particle. Very small particles scatter lights of shorter wavelengths better than those of longer wavelengths. From Table, it is clear that bluish lights have shorter wavelengths than reddish lights. When white light is incident on small suspended particles, more of the shorter wavelengths of light (bluish lights) get scattered. Very little of longer wavelengths of light (reddish lights) get scattered. So, the remaining light is reddish. The scattering of longer wavelengths of light increases as the size of the particles increases. Large particles scatter lights of all wavelengths equally well.

When smoke contains very fine particles of ash, the bluish lights get scattered, and the smoke looks milky blue. If the particles of ash are large, the entire light incident on it gets scattered. Then the smoke looks white. Clouds look white because of scattering by large drops of water in it.

Take water in a transparent container which is at least 18—20 cm long (or wide). Add two or three drops of milk to the water and shine a powerful torch through the water. Look from a side of the container. You will see that the colour of the milk—water mixture changes with distance from the torch. Near the torch, the colour is milky blue. And at the other end, the colour is orange or red. If you add a few more drops of milk, the colours become darker. Look at the face of the torch through the liquid. It will look reddish.

The suspended particles of milk scatter more of the bluish lights. This makes the mixture look bluish near the torch. As light travels down the container, the components of light left are mainly reddish in colour. So, the light looks orange or red. A similar thing happens in nature to make skies blue and the sun red when it rises or sets.

Sample Questions
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Question : 1

Which of the following are correct :

(a) Clouds look white because of scattering by large drops of water in it.

(b) When white light is incident on small suspended particles it gets scattered.

Right Option : C
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Question : 2

When smoke contains very fine particles of ash, the _______________ lights get scattered, and the smoke looks milky blue.

Right Option : A
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Question : 3

Which of the following are correct :

(a) When white light is incident on small suspended particles, more of the shorter wavelengths of blue light get scattered.

(b) When white light is incident on small suspended particles, Very little of longer wavelengths of blue get scattered.

(c) When white light is incident on small suspended particles, Very little of longer wavelengths of red get scattered.

Right Option : C
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Explanation
 
 


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