.05Å .2Å .8 3.2 12.8 50. 200. 800. 3200. 1.28µ --------+---------+------------+-------------+---------+-- ... RADIUM | | | | | γ | X RAYS | UNEXPLORED | ULTRAVIOLET | VISIBLE | RAYS | | | | | --------+---------+------------+-------------+---------+-- ... 1.28µ 5.µ 20. 80. 320. .128cm. .5cm. ... --+------------------+----------+--------- | | | HERTZIAN | INFRA RED |UNEXPLORED| ELECTRIC | | | WAVES ... --+------------------+----------+--------- 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 (λ_{max.}) 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 λ_{max.}×T=2890, and at 5000° C. (about the temperature of the sun) λ_{max.} 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. [Illustration: 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._)]