ANATOMY

Detailed explanation-1: -Pupils dilate (get larger) when room light is dimmed. Pupils are round and equal in size, in both bright and dim light. Pupils quickly and symmetrically constrict to a bright light directed into either of the eyes and when the bright light swings between the two eyes.

Detailed explanation-2: -When intense light rays reach your eye, the iris responds by constricting the pupil, thus protecting the retina and helping it process the incoming image better. The opposite occurs in low light when the iris dilates the pupil to allow as much light in as possible.

Detailed explanation-3: -In normal conditions, the pupils change size to let in the right amount of light. In the dark, they open wider or dilate to let in more light; in bright light, they get smaller or constrict to prevent too much light from getting in.