Eye to Eye
In its simplest sense, your eye is like a camera. Your eye has:
- A variable opening called the pupil
- A lens system, which includes the transparent covering called the cornea and a spherical lens
- A reusable "film" called the retina
- Various sets of muscles (The muscles control the size of the opening, the shape of the lens system and the movements of the eye.)
On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.
In the retina, sensory cells called rods and cones change the photons of light into electrical signals, which are then transmitted to and interpreted by the brain. The ability to focus the light on the retina depends on the shapes of the cornea and the lens, which are controlled by their inherent shapes, their stretchiness or elasticity, the shape of the eyeball and sets of attached muscles. So, when you look at something, muscles attached to the lens must contract and relax to change the shape of the lens system and keep the object focused on the retina, even when your eyes move; this is a complex set of muscle movements that is controlled automatically by your nervous system.
When you look at something, three things must happen:
- The image must be reduced in size to fit onto the retina.
- The scattered light must come together -- that is, it must focus -- at the surface of the retina.
- The image must be curved to match the curve of the retina.
As shown below, light passes through the cornea and pupil, is bent (refracted) by the lens, and comes to a point (focus) on the retina, where the image is formed.
To do all of that, the eye has a lens between the retina and the pupil (the "peep hole" in the center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens and the cornea work together to focus the image onto the retina.