Submitted by Nikki Lee, eighth grade, Cherokee Middle School

A. We encounter a huge range of light levels, from dark moonless nights to the glaring noon sun. But our eyes only work within a limited range -- too much light and the light-sensitive cells in our eyes are overwhelmed; too little light and the cells are not stimulated.

"We vary the amount of light that gets into our eyes so we can see in bright sunlight as well as in dark rooms, " said UW-Madison physiology professor Tom Yin.

Light enters the eye through the pupil, which is an opening in the center of the colored iris. Tiny muscles in the iris shrink or expand the pupil to control how much light gets through, Yin said.

This effect, called the pupillary reflex, is similar to adjusting the aperture on a camera. The pupil constricts in bright light and dilates in dim conditions.

The adaptation is most noticeable with an abrupt change of light levels, like walking from a well-lit area into a dim one or turning on a light in a dark room. For a moment, you cannot see clearly as your pupils adjust to the new conditions.

-- Produced in cooperation with University Communications.

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