Prisms - - Spectroscopy
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Volume 23, Issue 9, pp. 16-21

David W. Ball
A prism is an optical component that serves one of two major functions: it disperses light, or it modifies the direction (and sometimes polarization) of light (1). In some cases, a prism has more than one function. Prisms are usually transparent to the region of the electromagnetic spectrum being observed. Most people are familiar with transparent prisms that are used in the visible region of the spectrum.

Dispersing Prisms

As early as the 13th century, six-sided crystals of natural quartz were used to generate rainbows (2). The six sides complicated matters, however, making it difficult for experimenters to figure out what was going on. The prevailing view at the time was that the prism was adding color to the white light, which was the "pure" form of light. By the middle of the 16th century, high-quality triangular prisms were available (coincident with the rise of the Venetian glass trade), but scientists at the time still thought that the glass added color to white light.

A prism disperses light (that is, it spatially separates light by wavelength) if the light goes from one medium (say, the air) into another medium (say, a glass prism) at an angle other than 90░. The classic triangular prism has three nonparallel surfaces, so even if light passes through one surface at a 90░ angle, it will pass through a second surface at a non-normal angle.

Between 1666 and 1672, Isaac Newton performed his seminal experiments on white light with prisms. He was able to demonstrate conclusively that white light was in fact a composite of all colors, and that the prism itself only separated the colors. Newton was taking advantage of the fact that a prism disperses light; that is, it separates a range of wavelengths into its component wavelengths. A prism disperses as a result of two issues. First is Snell's law (3), which relates the angle of incidence θi to the angle of refraction θr for light impinging on a surface at some angle to the index of refraction of the two media involved:

n iĚsinθi = n rĚsinθr

where n i and n r are the indices of refraction of the media for the incoming light and the refracted light. The index of refraction is simply the ratio of the velocity of light in a vacuum c and the velocity of light in a given material v j:

n j = c / v j

As such, the slower light is when passing through a nonvacuum, the higher its index of refraction.

Figure 1
The second issue is the fact that the index of refraction of the prism material is not constant; rather, it varies with wavelength. As such, the amount of refraction is different for every wavelength of light, and so a prism acts to spatially separate the different wavelengths (that is, colors) of light. If the light going into the prism is a narrow image, the separation will be visible not far from the prism, before the light has gotten too dim (Figure 1).

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