Question

A pupil of our eye gets dilated when a person moves toward dark.

Answer 1

Yeah, it becomes shorter in light but inlarge in dim or no light.
The reason is because our eyes need a sufficient amount of light to make a clear image of the object in retina, so when the light becomes high the pupil contracts to avoid huge amount of extra light and when the light becomes low, it expands to take more amount of light.
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Answer 2

To see more clearly our pupils of our eyes gets smaller and smaller so that in dark also we can see all the things clearly. This process takes 4 to 5 mins.