35.5.1 Color

It has been known since antiquity that irregularly shaped glass and crystals cause sunlight to be broken into various colors. A common idea was that the glass or crystal somehow altered the properties of the light by adding color to the light. Newton suggested a different explanation. He first passed a sunbeam through a prism, producing the familiar rainbow of light. We say that the prism disperses the light. Newton’s novel idea, shown in FIGURE 35.17a, was to use a second prism, inverted with respect to the first, to “reassemble” the colors. He found that the light emerging from the second prism was a beam of pure, white light.

FIGURE 35.17 Newton used prisms to study color.

The figure illustrates the dispersion and reassembling of white light through a glass prism.
The figure illustrates the dispersion and reassembling of white light through a glass prism and an aperture that selects only the white light.
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But the emerging light beam is white only if all the rays are allowed to move between the two prisms. Blocking some of the rays with small obstacles, as in FIGURE 35.17b, causes the emerging light beam to have color. This suggests that color is associated with the light itself, not with anything that the prism is doing to the light. Newton tested this idea by inserting a small aperture between the prisms to pass only the rays of a particular color, such as green. If the prism alters the properties of light, then the second prism should change the green light to other colors. Instead, the light emerging from the second prism is unchanged from the green light entering the prism.

These and similar experiments show that

  1. What we perceive as white light is a mixture of all colors. White light can be dispersed into its various colors and, equally important, mixing all the colors produces white light.

  2. The index of refraction of a transparent material differs slightly for different colors of light. Glass has a slightly larger index of refraction for violet light than for green light or red light. Consequently, different colors of light refract at slightly different angles. A prism does not alter the light or add anything to the light; it simply causes the different colors that are inherent in white light to follow slightly different trajectories.