Question

Why does the pupil of our eye contracts in bright light and expands in dim light?

Answer 1

Because if we see in dim light so Pupil will dilate.And in bright light it expands.If large amount  entering your eyes is much more than required for your Brain.So it will contract

Answer 2

Eyes cannot withstand either too bright light or too dim light.

To decrease the intensity of the light entering, the pupil contracts when eyes are exposed to bright light.

To allow more light to pass through, the pupil dilates when eyes are exposed to dim light.

This a way to adjust the amount of light passing through the eye.