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
[Illustration: FIG. 1.--Schematic representation of various types of
radiation to form a wide continuous spectrum.]
.05A .2A .8 3.2 12.8 50. 200. 800. 3200. 1.28 mu
--------+---------+------------+-------------+---------+-- ...
RADIUM | | | | |
[gamma] | X RAYS | UNEXPLORED | ULTRAVIOLET | VISIBLE |
RAYS | | | | |
--------+---------+------------+-------------+---------+-- ...
1.28 mu 5. mu 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 ([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