David W. Ball

When a person says the word "light," a listener — even a spectroscopist — usually interprets that as meaning "visible light." That's not unexpected, because throughout most of recorded history the only light that we recognized was light that we could see. However, that changed in the 1800s when it was finally realized that light was a more general phenomenon. Although currently the term "light" refers to a lot more than just visible light, it's more common to use the phrase "electromagnetic radiation" when referring to any form of light. The collective range of possible lights is called the electromagnetic spectrum.

Properties of Light — A Quick Review

Light has both particle and wave properties, as has been discussed in a recent column (1). Because a photon of light has a certain energy E and momentum p, it acts as a particle:

Because light's properties can be described by values of wavelength λ, frequency υ, and wavenumber υ tilde (see Figure 1), it acts as a wave. As with any wave, the velocity of light is equal to the product of its frequency and wavelength:

Figure 1

Unlike that of other waves, the speed of light is a constant for a given medium. In a vacuum, the speed of light is approximately 3.00 × 10⁸ m/s — about seven and a half times around the Earth every second. (See Figure 2 for an example of the speed of light.)

Because the speed of light is a constant, there is a simple relationship between the wavelength of a light and its frequency. For visible light, wavelengths range from 400 to 700 nm, which correspond to frequencies of 7.5 × 10¹⁴ to 4.3 × 10¹⁴ waves per second. The energy and momentum of each photon of visible light can be calculated from the earlier equations.

Figure 2

Other Than Visible Light

In 1800, British astronomer William Herschel was measuring the effect of various colors of light on a thermometer, using a prism to disperse light from the sun. Upon putting the thermometer past the red light, he noted an even larger increase in temperature than when the thermometer was bathed in visible light. It was obvious that there was "light" beyond the color red; this light was eventually termed "infrared" light — literally, "below red" light.