Light is electromagnetic energy that excites the retina, so it's important to understand how we see light. We'll begin with how the eye functions and then discuss factors that affect our seeing including contrast, size, brightness, time and the effects of aging.

Light enters the eye through the cornea, the clear outer layer of the eye then the pupil, and then through the lens of the eye. The lens has muscles that adjust its shape to focus the light much like a camera lens does. A fluid called the vitreous humor fills the inside of the eye. The light passes through this viscous fluid to the rear wall, called the retina.

The retina consists of light sensitive cells called rods and cones. The rods provide night or low light vision. The cones provide daylight or bright light vision and color vision. There are actually three different kinds of cones differentiated by their pigment. The combinations of these pigment cones allow us to see colors. When a person is color blind, they are lacking a certain type of pigment.

There's a small pit in the center of the retina called the fovea that has only cones, so it provides the most accurate bright light vision. Your eyes move continually to try to keep an image concentrated on the fovea. Of course this is useless in dim light. To improve vision in dim light it's better to look to the side of an object so the image focuses on rods instead of cones.

The rods and cones convert the light images into neural signals. The optic nerve transfers these signals to the brain for processing. Rods and cones respond differently to the electromagnetic spectrum.

The result is that, in bright light, a red and a blue object might look equally bright, but under dim light the blue object will appear brighter. Notice that the two curves peak at different wavelengths. This means that the eye under normal daytime illumination is most sensitive to light in the yellow-green hues. In low light vision, the eye is most sensitive to light in blue hues and is almost blind to red.