The opening at the center of your eye allows light to enter it, enabling you to gather information about the world around you, but the opposite is true, too—insights can be gained about what is going on in your brain based on the behavior of your pupils. The pupil is the opening at the center of the eye that appears as a black dot surrounded by the colored part of the eye, the iris. The iris is a muscle in the eye that functions like the diaphragm of a camera. The iris responds to the amount of light entering the eye by adjusting the size and diameter of the pupil (the aperture), in order to allow the appropriate amount of light into the eye (the camera). Light travels through fluid in the eye and is then absorbed at the back of the eye, in an area known as the retina. The retina is covered in specialized cells called photoreceptors (think of this part of the eye as the film of a camera where the picture is captured). Photoreceptors gather information from the light and send it to the brain to be processed into the image you see.

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