- Human vision is possible due to the ability of the eye to sense light. When a light source such as the sun strikes an object, the object reflects certain wavelengths of light and absorbs others. Things that are white in color reflect all wavelengths of light, while black objects absorb all wavelengths. Colored objects absorb certain wavelengths but reflect those of the color that the objects appear. When the light reflected by objects enters the eye, it passes through a transparent layer called the cornea and then through the pupil, which is essentially a hole in the middle of the eye. Then a lens behind the pupil focuses the light on the back of the eye, which is known as the retina.
- The retina is a sensitive area in the back part of the eye containing millions of cells known as rods and cones. Rods are good at sensing dim light or light that is devoid of color, while cones are specialized at sensing color. If too much light enters the eye, the rods and cones can be overwhelmed, which can result in eye damage. A ring of muscles in the eye called the iris (the colored part of the eye) flexes and contracts to alter the size of the pupil and therefore the amount of light that enters the eye. In conditions with ample light, the pupil gets smaller to prevent the retina from being overwhelmed, while the pupil dilates in dark conditions to take in as much light as possible in order to sense objects.
- After light strikes the rods and cones, they initiate a chemical reaction that turns the image focused on the retina into electrical impulses that are then sent to the brain through the optic nerve. When the impulses reach the brain, it translates them into the images you see. As light enters the eye, the lens bends it, creating an upside down image on the retina. The brain must flip the image back so that we see things properly.
