How does the eye work?
Answer
The eye contains special cells that receive incoming light energy and convert it into electrical impulses. These electrical impulses are then relayed on the optic nerve to our brain. Next, let's look at the parts of the eye. The cornea covers the entire front part of the eye. Its convex shape allows it to bend or refract incoming light into the lens. The lens is a transparent, biconvex, flexible structure that focuses the light onto the retina. If you want to see something close, the lens is made to be thicker. If you want to see something further away, the lens is made to be thinner. These changes increase or decrease the refraction of incoming light onto the retina. Changes of the thickness of the lens are done by the ciliary muscles which change with contraction and relaxation and are connected to the lens by suspensory ligaments.
The iris is the colored part of the eye. It is the muscular structure that regulates the size of the pupil, the black opening in the middle of our eye that controls the amount of light that enters our eye. The iris does this by expanding or contracting depending upon the amount of light in our surroundings. When there is a lot of light, the iris expands so that the pupil becomes smaller and so less light will enter. When there is less light, the iris contracts making the pupil larger so that more light can enter. The color of our iris depends on how much melanin is present. This is determined by genetics. Brown-eyed individuals have more melanin than blue-eyed individuals.
Light enters the eye through the cornea and passes to the pupil onto the lens. The lens refracts light through the vitreous humor onto the retina. The retina at the back of the eye contains a thin line of cells called photoreceptor cells. There are two types of photoreceptor cells: the rods and the cones. They each have distinctive shapes and are sensitive to different kinds of light. We have approximately 120 million rod cells that allow us to see black, white, and shades of grey. The rods enable us to distinguish shapes. As rods are sensitive to reduced light, they provide us with nocturnal night vision. At 7 million cone cells, we are specifically allowed to see color. The cells all work together to see the finest in detail. Where the optic nerve connects to the eye, there are no rods nor cones. This is known as the blind spot.
When light enters the eye, it travels through the cornea, through the pupil, refracts through the lens as it crosses the vitreous humor, and focuses an image onto the retina. Light-sensitive rods and cones react to different wavelengths and trigger nerve impulses. These nerve impulses are carried through the optic nerve to the thalamus where it synapses and continues to the primary visual cortex of the brain. This is where impulses are organized and interpreted to give us sight as we know it.
Or you may prefer the link to the video (see How the eye works).