Page:Elementary Text-book of Physics (Anthony, 1897).djvu/459



362. Effects of Radiant Energy.—It has been stated that the solar spectrum, whether produced by means of a prism or by a grating, may, under certain conditions, give rise to heat, light, or chemical changes. It was formerly supposed that these were due to three distinct agents emanating from the sun, giving rise to three spectra which were partially superposed. Numerous experiments show, however, that, at any place in the spectrum where light, heat, and chemical effects are produced, nothing which we can do will separate one of these effects from the others. Whatever diminishes the light at any part of the spectrum diminishes the heat and chemical effects also. Physicists are now agreed that all these phenomena are due to vibratory motions transmitted from the sun, which differ in length of wave, and which are separated by a prism, because waves differing in length are transmitted in the substance of the prism with different velocities. The effect, produced at any place in the spectrum depends upon the nature of the surface upon which the radiations fall. On the photographic plate they produce chemical change, on the retina the sensation of light, on the thermopile the effect of heat. Only those waves of which the wave lengths lie between 3930 and 7600 tenth metres affect the optic nerve. Chemical changes and the effects of heat are produced by radiations of all wave lengths.

To produce any effect the radiations must be absorbed; that is, the energy of the ethereal vibrations must be imparted to the substance on which they fall, and cease to exist as radiant energy. The