CHAPTER II

Chapter 2808 wordsPublic domain

LUMINESCENCE AND INCANDESCENCE

Modern physical theory supposes that light is a succession of wave pulses in the ether caused by vibrating electrons. The light to which we are most accustomed--sunlight, electric light, gaslight, etc.,--is due to electrical phenomena connected more or less directly with the high temperature of the source of the light. Every solid body above the temperature of absolute zero is giving off waves of different wave-length ([lambda]) and frequency ([nu]) but of the same velocity ([upsilon]), in vacuo, 180,000 miles, or 300,000 kilometres a second. In fact, [upsilon] (a constant)=[lambda]_{[nu]}, so that it is only necessary to designate the wave-length in order to characterize the waves. This is radiant energy or radiant flux.

As everyone knows, the long waves given off in largest amount from objects at comparatively low temperatures give the sensation of warmth. As we raise the temperature, in addition to these longer heat waves, those of shorter and shorter wave-length are given off in sufficient quantity to be detected. At 525 deg. C., rays of about [lambda]=.76 mu in length are just visible as a faint red glow to the eye. As the temperature increases still shorter wave-lengths become apparent, and the light changes to dark red (700 deg.), cherry red (900 deg.), dark yellow (1100 deg.), bright yellow (1200 deg.), white-hot (1300 deg.) and blue-white (1400 deg. and above). Above [lambda]=.4 mu the waves again fail to affect our eye, and, although they are very active in producing chemical changes, we have no sense organs for perceiving them. Thus, a white-hot object liberates radiant energy or flux of many different wave-lengths corresponding to what we know as "heat, light and actinic rays." All can be dispersed by prisms of one or another appropriate material to form a wide continuous spectrum, such as that indicated in Fig. 1. Radiant energy of [lambda]=.76 mu to [lambda]=.4 mu, evaluated according to its capacity to produce the sensation of light, is spoken of as visible radiation or luminous flux.

Below the infra-red comes a region of wave-length as yet uninvestigated, and beyond this may be placed the Hertzian electric waves of long wave-length used in wireless telegraphy. Above the ultra-violet comes another region as yet uninvestigated, and then Roentgen rays (X-rays) and radium rays, of exceedingly short wave-length. These last types need not concern us except in that we may later inquire if they are given off by luminous animals. The shortest of the ultra-violet are known as Schumann and Lyman rays. These relations are brought out in Table 2.

TABLE 2.

_Wave-lengths of Various Kinds of Radiation_

Wave-lengths of light are usually given in Angstrom units. One micron ( mu)=.001 mm.=1000 millimicrons ( mu mu)=10,000 Angstrom units (A) or tenth metres=10^{-10} metres or 10^{-8} centimetres. The entire scale of wave-lengths extends from 10^6 to 10^{-9} centimetres.

Hertzian electric waves (upper limit not reached) above 12 km. to .16 cm. Unexplored region .16 cm. to 310 mu Infra-red 310 mu to .76 mu Visible light 7600 A to 4000 A Ultra-violet 4000 A to 320 A Unexplored region 320 A to 12 A X-rays 12 A to 0.2 A Radium [gamma] rays 0.2 A and shorter

.05A .2A .8 3.2 12.8 50. 200. 800. 3200. 1.28 mu --------+---------+------------+-------------+---------+-- ... RADIUM | | | | | [gamma] | X RAYS | UNEXPLORED | ULTRAVIOLET | VISIBLE | RAYS | | | | | --------+---------+------------+-------------+---------+-- ...

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 ([lambda]_{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 [lambda]_{max.}xT=2890, and at 5000 deg. C. (about the temperature of the sun) [lambda]_{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