The human eye sees light with the pupil and the retina. Located inside the retina are two types of special cells called photoreceptor cells. There are two varieties of photoreceptor cells known as cones and rods.
Rods are more sensitive to light than cones, and they help to see lights, especially at night time. Cones are useful in distinguishing different colors of the light the human eye sees. While the majority of people have three different kinds of cones in their eyes, colorblind people are an exception. Those diagnosed as colorblind have only two cones in their eyes, not three. The pupil gets larger or smaller, depending on the brightness of the light, to let enough light particles (known as photons) into the eye. The photons are then absorbed by the molecules in the rods and cones located in the eye. The light energy is then used to convert the molecules located within that cell into a higher energy state. This higher energy causes a response to be sent through the optic nerve to the brain. This response causes the brain to recognize the image seen as "light." While the human eye can see many different types of light, there are also many types of light that the human eye does not see. Microwaves, x-rays and gamma rays are some types of light that are undetectable to the human eye.