The cornea is a transparent, dome-shaped structure that covers the front of the eye and helps to focus light. The RETINA is a thin layer of tissue at the back of the eye that contains cells that sense light and send signals to the brain. The IRIS is the colored part of the eye that controls the amount of light entering the eye. The LENS is a transparent structure behind the IRIS that helps to focus light onto the RETINA. The PUPIL is the dark circle in the center of the IRIS that allows light to enter the eye.