In Depth: Eye Cross-section
When light strikes the eye, the first part it reaches is the cornea, a dome positioned over the center of the eye. The cornea is clear and refracts, or bends, the light passing through it.
Light then reaches the pupil and the iris. These parts of the eye are responsible for regulating the amount of light that gets through. Too much or too little light can hinder one’s vision. The muscular iris moves to shrink the pupil if there is too much light and widen it if there is not enough. This is an involuntary function, controlled by the brain.
Deeper inside the eye is the lens, which further refracts light and helps create a more precise image. The shape of the lens can be manipulated to help the eye see things better depending on the proximity of the object being viewed. The lens flattens to properly focus light received from distant objects and becomes rounder for nearer objects. This is also an involuntary action. The inability to do this properly is what causes near- or farsightedness.
Once past the lens, light strikes the millions of photoreceptors cells in the retina. There are two types of photoreceptors, rods and cones, which are named for their shape. Rods work in less light and create black-and-white images, and cones work in bright light and allow for color vision.
There are three types of cones: one sees red, one sees green, and one sees blue. Lack of one or all of these is what causes color blindness. A lack of the green or red cones (which results in red-green color blindness) is more common than lack of blue cones or lack of any cones at all.
The retina’s photoreceptors react to the light that hits them and cause nerve impulses to be sent to the brain via the optic nerve. The brain interprets and classifies the visual information.
The “white of the eye” is the tough outer shell called the sclera. Inside the eye is a fluid called the vitreous humor, a jelly-like substance that helps give the eye its shape. Another fluid of the eye is the aqueous humor, which lubricates the iris.