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Refraction of Light

When a scale is immersed in a beaker filled with water in a slanting position, the scale appears to be bent at the surface of water. Why is it so?
 
This is because light rays from the scale immersed in water, first travel in water and then through air.

Even though air and water are transparent media, there is difference in their optical densities.
When light passes from one medium to another having a different optical density, its direction changes. This takes place at the surface where the two media meet.
 
This phenomenon is called ‘refraction of light’ (Figure 9.17). As a result of refraction, the stick appears to be bent.
 

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Figure 9.17 Refraction of Light

 

The change in the direction of the path of light when it travels from one transparent medium to another transparent medium of different optical density is called refraction of light.
 
Why do Light Rays Bend?
Light travels at different speeds in different optical media. Its speed while travelling through vacuum is  3 × 108 ms1 (its speed while travelling through air is slightly less) and its speed in water is 2.25 × 108 ms1, and in glass it is 2 × 108 ms1.
 
This difference in the speed of light while travelling from one medium to another causes the light rays to bend or change the direction of its path at the surface of separation of the two media.

Denser and Rarer Medium

When light passes from one medium to another and if the speed of light decreases, the second medium is said to be optically denser with respect to the first medium. Conversely, if the velocity increases, the second media is said to be optically rarer with respect to the first medium.

It has been experimentally observed that
  1. When a ray of light travels from a rarer medium to a denser medium (say, from air to water), the refracted ray bends towards the normal, i.e., r < i as shown in Figure 9.18. The deviation of the ray is δ = i − r.
  2. When a ray of light travels from a denser medium to a rarer medium (say, from glass to air), it bends away from the normal (i.e., r > i) as shown in Figure 9.19. The deviation of the ray is δ = r − i.

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Fig. 9.18 Refraction of Light from Rarer to Denser Medium

 

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Fig. 9.19 Refraction of Light from Denser to Rarer Medium





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