The eye is a complex optical system – very similar to a camera. Vision begins when light enters the eye through the cornea, a powerful focusing surface. From there, it travels through clear aqueous fluid, and passes through a small aperture called the pupil. As muscles in the iris relax or constrict, the pupil changes size to adjust the amount of light entering the eye. Light rays are focused through thelens, and proceed through a clear jelly-like substance in the center of the eye called vitreous, which gives it form and shape. When light rays finally land on the retina, the part of the eye similar to film in a camera, they form an upside-down image. The retina converts the image into an electrical impulse that travels along the optic nerve to the brain, where it is interpreted as an upright image.