Britannica Sources used in this answer

How do cat pupils change in bright and dim light?

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A Maine Coon longhair cat with cream and white fur sits beside a tabby shorthair cat in a garden with green foliage in the background. — cat Two kinds of domestic cats.

Cats possess vertical slit pupils that allow for precise control over the amount of light entering the eye, an adaptation that supports their activity during dawn and dusk.1

In dim light: The pupils expand (dilate) to open wide, allowing as much light as possible to enter the eye. This significantly improves their night vision, which is essential for hunting and navigating in low-light environments.1
In bright light: The pupils constrict to a very narrow vertical slit. This reduction in the aperture limits the amount of light reaching the retina, which prevents damage to sensitive tissues and maintains optimal vision.1

This dynamic adjustment is governed by the muscles of the iris, enabling the cat to adapt rapidly to changing environmental conditions.12