A triangular prism refracts light twice as it passes through, once when it enters the prism and again when it exits. In both cases, as seen in the diagram below, the light is refracted downward. This results in white light being broken up into spectral colors. The braking up of white light into its constituent colors is dispersion.

Prism diagram here 

Isaac Newton was the first to study dispersion in detail. He broke up white light with a prism, and then put another prism in the red portion, to see if it would disperse as well. It did not, so he theorized that white light was composed of independent colors, each of which are distinct.

White light is dispersed because different colors are affected differently by the passage into glass. Specifically, the different colors travel at different speeds through the glass (Because of this, each color has its own index of refraction). Red light travels at the fastest speed through the glass, and so is bent the least by its passage. Violet light travels at the slowest speed, and so is bent the most. As it turns out, the speed is related to the wavelength (by v = fl). This indicates that the color with the highest speed (red) has the longest wavelength, while the color with the slowest speed (violet) has the shortest wavelength. Because frequency is inversely proportional to wavelength, we can also determine the relative frequencies. Red would have the lowest, while violet has the highest.

An easy way to remember the spectral colors is to use the mnemonic ROYGBV.

RedOrangeYellowGreenBlueViolet

                        Longest l                                 Shortest l

                   Lowest f                                   Highest f

As it turns out, the energy of light is directly related to its frequency (more on this later). We see colors because each has a different energy, and so affect cone cells on our retinas differently. Although humans can detect thousands of different color hues, we only have three different types of cone cells on our eyes. One type of cone (we’ll call this the “red cone”) detects light from the red and orange section, the lower-energy section, of the color spectrum. A second type of cone detects middle-energy light, that from the yellow and green parts of the spectrum (the “green cone”). The third type detects high-energy light, from the blue and violet portions. With these three types of cones, we can see all of the different hues because of the way color mixes.         

We can ask a simple question. Why is a ripe apple red in color? It must have something to do with the chemicals in the skin of the apple. But how do these chemicals affect light?

We have seen that white light is made up of the spectral colors of light. When white light is incident on the apple, most of the spectral colors are absorbed out. The only one not absorbed is red. The red is reflected from the apple to our eyes, and so the apple is red in color. When the light from the apple lands on our retinas it affects only the “red” cones. These cones send a signal to our brain, so we see it as red.

Information provided by: http://www.whitewater.k12.wi.us