Fig. 1.—Schematic representation of various types of radiation to form a wide continuous spectrum.
The total radiant energy which a body emits is a function of its temperature and for a perfect radiator, or what is known as a black body, the total radiation varies as the fourth power of the absolute temperature, T. (Stefan-Boltzmann Law). The radiant energy emitted at different wave-lengths is not the same but more energy is emitted at one particular wave-length (λmax.) than at longer or shorter ones, depending also on the temperature. If the various waves are intercepted in some way, their relative energy can be measured by an appropriate instrument and spectral energy curves can be drawn, showing the distribution of energy throughout the spectrum. [Fig. 2] gives a few of the curves, and it will be noted that the maximum shifts toward the shorter waves the higher the temperature. In fact, for a black body λmax.×T=2890, and at 5000° C. (about the temperature of the sun) λmax. lies within the visible spectrum. In gas or electric lights it lies in the infra-red region. The area enclosed by these spectral energy curves represents the total energy emitted, and, knowing this and the area enclosed by the curve of visible radiation, it is easy to determine how efficient a source of light is as a light-producing body. We shall inquire more fully into this question in Chapter III, in considering the efficiency of the firefly as a source of light.
Fig. 2.—Distribution of energy throughout the spectrum of the sun, electric arc, and gas light (after Nichols and Franklin). Ordinates show the relative intensities of different wave-lengths emitted. The notches in the curve represent absorption bands and the dotted line represents what the radiation from the sun would be if no selective absorption occurred. V=violet and R=red end of visible spectrum. (Courtesy Macmillan Co.)
A body which emits light because of its (high) temperature is said to be incandescent and we speak of temperature radiation. We know, however, of many cases where substances give off light at temperatures much below 525° C. They do not follow the Stefan-Boltzmann law. The light emission is stimulated by some other means than heat. Such bodies we speak of as luminescent, and in this category belong all luminous animals. The distinction between light and luminescence was first pointed out by Wiedemann (1888). It is usual to classify luminescences, according to the means of exciting the light, into the following groups: