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plzz explain principle of reversibility of light?

Principle of reversibility of light states that, if we reverse the path of light when it travels from 1 medium of refractive index n1 to another medium of refractive index n 2, then it will retrace the entire path through which it travelled before.Let the ray of light travel from air to glass as shown in the figure.

 By applying the principle of reversibility of light, the light makes the same angle, iAIR, with the normal in air as it did originally when travelling in the opposite direction.

 

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 The principle that if a beam of light is reflected back on itself, it will traverse the same path or paths as it did before reversal. 
 

The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.

Hence:

Using Snell's Law,

sin i
sin r

= 1n2

sin r
sin i

= 2n1

1n2=

1 
2n1

It follows that

1n2=

1 
2n1
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If direction of light ray  is reversed then light ray retraces its path.

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