Color is as much a part of the human vision system as it is of the physics of light and matter. Without color vision, color would be merely a variation of light waves. For those animals that do not see color, color as we see it does not exist for them. It is the ability of the human vision system to detect and separate the various frequencies of light waves that allows us to observe different colors.
The blue and green light waves from the sunlight are absorbed by the apple's skin.
The Sun APPLE  Eye  Red light is reflected The Apple appears red

We see the apple as red because the image of the apple's red skin affects the red sensitive vision cells of the eye more than it affects the blue and green vision cells. We then identify the apple's color as being "red".

Vision cells are positioned on the retina, located on the back inside surface of each eye. The cells that allow us to see color are called "cones". There are several types of cone cells distributed across the retina. There are cones that are especially sensitive to red. Others cone cells are especially sensitive to green or blue.

The colors red, green and blue are also the primary colors of light. That means that all other colors can be recognized by the eye as mixtures of different intensities of red green and blue.

THE COLOR spectrum

A glass prism can be used to separate white light into its various wavelengths or frequencies of light. This shows that white light from the sun is actually a mixture of color.

White Light
	Prism

Different colors of light have different wavelengths.

The wavelength of color light is measured in nanometers.
	red
	orange yellow
	green	The numbers shown here are the number of nanometers between each wave of light.
	blue
	violet

How small is the distance between each wave of light? Well there are one billion nanometers in one meter. So the waves of light are extremely small, but they make all the difference in the colors that we see.

It is this difference in wavelengths of light that the human eye detects as different colors.

UN-MIXING COLOR LIGHT

When we look at a yellow light bulb, we do not readily see the yellow as a mixture of red and green light. This diagram shows how light from a yellow lamp would appear if it were broken up into its component colors by using a prism.

How does a prism "un-mix" color illumination? The prism can do this because it refracts or bends the light passing through it. The prism bends the green part of the illumination at a different angle than red illumination. This causes the colors to shine on different areas of a screen or wall. In this example, the light from the yellow lamp is broken up into its component colors. This shows us that the yellow illumination is actually a mixture of green and red light.

Note: Colors such as magenta and brown are not in the spectrum produced by a prism. However, magenta illumination can be produced by mixing red light and blue light. Brown light is actually a low intensity yellow light or a low intensity of orange light depending on the type of brown.

Red and blue light make magenta		Yellow and orange dim to brown

COLOR VISION ADAPTATION

• Our eyes automatically adjust to the quality of a light source in order to match a standard of viewing that is part of the human eye-brain system of perception. We look at a pink flower in morning sunlight and it appears pink. We view the same flower at sunset and it looks the same pink color. We look at the same flower under cloudy skies and it still appears "pink". But if we were to photograph this flower under these three conditions, the pink of the flower may vary greatly in the photo. Its pink petals may be tinged with blue, orange, green, gray or some other hue. This is caused by the quality of the light. Even though the light source may change, our brains seek consistency. Modern digital cameras often have a "white balance" feature that adjusts the image so that the colors in the final photo appear acceptable to us.  

Much of what we see as color is a combination of human vision and the science of light.