Light travels in waves of different wavelengths. Light is emitted from sources such as the sun, lightbulbs, and fire in many different wavelengths, however the human eye can only detect certain kinds. The eye is capable of sensing light of wavelengths ranging from about 400 to 700 nanometers.
The wavelength of the light determines its color, for example light that has a wavelength of 520 nanometers appears green. Any color that is of a single wavelength (such as 520 nanometers) is called monochromatic. Scientists have displayed all of the monochromatic colors in what is called the visual spectrum and for this reason monochromatic colors are often called spectral colors. The spectrum to the left shows the many different wavelengths of light: the further to the right on the graph the longer the wavelength. These colors may look familiar to you, as they are often simplified to the colors of the rainbow: ROYGBIV. Notice that at both the beginning and the end of the graph the spectrum fades to black because the human eye is no longer capable of processing light outside of the 400-700 nanometer range.
But these are not all of the colors that we are able to see. By mixing light of different wavelengths, it is possible to create different colors! (You will learn more about mixing colors in Color Changes ) Colors formed by mixing different wavelengthis of light are called polychromatic colors. The colors we can see because of mixing are shown on something called a Chromaticity Diagram. This shows all of the spectral colors along the edges and the colors formed by mixing them in between. Notice that white is in the center of the diagram; white light is actually a mixture of all wavelengths of light.
Color is often classified based on the chromaticity diagram model. The HSL Classification System defines colors in terms of hue, saturation, and lightness. Hue is the closest wavelength of light to the color on the chromaticity diagram. Saturation (also called purity) measures how close the color is to the wavelength on the diagram. A color of a 520nm hue and 100% saturation would be pure 520nm light so would appear green. A color of a 520nm hue and 0% saturation would be in the center of the diagram so would appear white or gray. The final number for color classification is lightness, which measures the intensity of the color. Lightness of 0% would be black because no light would be reflected. Lightness of 100% would be white because all light would be reflected. Lightness between these extremes creates different shades of colors.
The Primary Colors
The human eye only has 3 color channels. In other words, our eyes are not able to distinguish the particular wavelengths of light and can only use three different sensors to determine what color an object should appear.
We see the millions of colors around us because of the way that the color channels send signals to the brain. The color channels are called Red, Green, and Blue. Each channel is sensitive to a certain range of light. At 520nm the green channel is very sensitive but the blue channel is only moderately sensitive and the red channel is barely sensitve at all. When 520nm light strikes the eye, a very strong "green" signal, a moderate "blue" signal, and a weak "red" signal are sent to the brain. The brain combines these three signals to determine that the light is 520nm, making the light appear green.
Because red, blue and green are the three colors that the human eye can see, these are called the primary colors of light. Any color visable to humans can be described as a combination of signals from the red, green, and blue, so the RGB Color Classification System was created. This system defines a color in terms of the amount of stimulation it creates in each of the three color channels. An activity using the RGB Color Classification System is in the next section, Color Changes.