Light enters a plane-parallel block of glass from the vacuum of space and leaves again into space through the opposite side (e. g., in a spaceborne instrument). what is the speed of light after it leaves the glass?

This question is based on the simple principle of the refraction.

Refraction is the optical phenomenon in which the light ray will change its direction when fall obliquely on a refracting surface due to the change of velocity of light in the refracting medium.

Greater  the refractive index,the lesser will be the velocity of light in that medium.

As    refractive index

Here is the velocity of light in first medium and is the velocity of light in second medium.

In case of light travelling from space to the medium we may write

                       where c is the velocity of light.

    So as long as the medium is same ,there will be no change in the velocity of light.

As per the question light was initially in space. After then the light entered into glass medium whose refractive index is more as compared to space.Hence velocity of light will be decreased .

Now the light escaped through the glass.so it again enters the same medium i.e space. Hence the velocity of light will be increased now and will be equal to 'c' now .

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