Light reflects off the object we’re looking at and enters the eye through the cornea, a clear, thin, dome-shaped tissue at the very front of the eye. The cornea has a curvature to it and covers the eye, kind of like a crystal covering the face of a watch. “When rays of light enter the eye, they’re sort of parallel to each other,” says Rosen. “But as they pass through the cornea, they bend and start to converge, almost coming to a point on the retina.” From there, light travels through a clear fluid, called the aqueous humor, which fills small chambers behind the cornea, nourishes the eye and helps retain pressure to help the eye retain its shape.
As the light continues, it passes through an opening called the pupil , that black dot at the center of the eye. The pupil is surrounded by the iris, the colored part of the eye. It’s the iris’s job to control how much light the pupil lets into the eye. When there is bright light, the iris uses muscles to change the size of the pupil (making it contract) to let in less light. When there is low light, the iris opens up the pupil, making it wider, to let in more light.
Next, the light penetrates the lens, a transparent structure that works with the cornea to bend light and focus it onto the retina, which is located in the back of the eye. “The lens accumulates proteins as we age, causing a cloudy lens, or cataracts,” says Jaclyn Haugsdal, M.D., clinical assistant professor of ophthalmology and visual sciences at the University of Iowa Carver College of Medicine. Small elastic muscles, known as ciliary muscles, which are attached to the lens, help it change its shape in order to focus at various distances. When these muscles contract, the curvature of the lens increases, allowing us to see objects that are close up. When these muscles relax, the lens becomes flattened, helping with long-range vision.
The large space behind the lens, in the back portion of the eye, is filled with a clear, gel-like substance, called the vitreous. The gel helps keep the space in the middle of the eye clear so light can reach the retina. “It also provides some elasticity to the eye, helping it keep its shape,” says Rosen. “For example, rubbing your eyes causes your eye pressure to spike, but it returns to normal when you stop rubbing. Or, if you poke your eye, it doesn’t automatically deflate because this kind of elastic material fills the eye and absorbs the impact, preventing it from causing much damage.”