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Reversibility of light
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Consider the light to be travelling in the opposite direction, from the glass block into air.

Definitions

By 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.

Have a go!

In the top ray diagram, see how iGLASS increases as you increase the value of iAIR.

Now

sin iGLASS / sin iAIR = GLASSnAIR
= 1 / AIRnGLASS
= 1 / 1.50

Have a go!

Slowly increase the value of iGLASS and see how iAIR increases.

[ Refraction applet ]
[ Refraction applet ]
 

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