Light has different wavelengths, which we see as colors. The range of wavelengths we see is called the visible spectrum. We separate the colors of the spectrum by DISPERSION.
Light waves are just one type of electromagnetic wave. They belong to an electromagnetic spectrum that includes radio waves, X-rays, and gamma rays. The visible spectrum is the only part the human eye can see. To our eyes, the colors in the visible spectrum range from violet at one end to red at the other.
The light-sensitive cells in the human eye react to just three types of light: red, green, and blue wavelengths. These are the three primary light colors. If all three types of wavelength enter the eye with equal strength, we see white light. When just red and green light are present, we see the mixture as yellow.
Different wavelengths of light blend to produce millions of shades of color. The human eye is able to pick out over 10 million of them—some of which can be shown by a COLOR TREE. The amount of color we see depends on how much light there is. In dim light, we see no colors at all, only shades of gray.
When white light shines through a specially shaped piece of glass called a prism, it is separated into its different wavelengths by dispersion. The wavelengths show up as a range of colors called a spectrum. English scientist Isaac Newton first used a prism to disperse sunlight in the late 1600s.
Rainbows appear when there are water droplets in the atmosphere and bright sunshine at the same time. The droplets act like tiny prisms, refracting and reflecting the sunlight, and dispersing it into the colors of the spectrum. To see a rainbow, you have to be standing at a particular angle to the water droplets and the Sun.
A color tree is one way of grading or classifying colors. Using a color tree, it is possible to describe and then match a particular shade of color (of paint or fabric, for example).