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

Dear student,
Please find the answer to your query in the link below :

https://www.meritnation.com/ask-answer/question/we-know-that-when-we-enter-a-dimly-lit-room-the-iris-expan/light/1030539

Hope this will clear your doubt about Refraction of light.
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