ABSORPTION OF WAVES. Waves of any kind in any medium carry energy with them; and, if the energy decreases, the medium is said to absorb it or to exhibit "absorption." Thus, if white light falls upon red glass, i.e., if ether waves which affect the normal human eye with the sensation "white" are incident upon glass which appears red to the same eye, all the waves except those which produce the sensation red are absorbed by th.2 glass, while the others are transmitted. Bodies differ greatly in the qual ity and quantity of their absorptive power; but it is a general law that the absorptive power of a body equals its emisAive power under the same conditions. (See RAmATiox.) Absorption is due to the presence in the pure medium carrying the waves of some portions of matter whose own natural period of vibration is the same as that of the period of the waves; and, therefore, these portions of matter are set in vibration by "reso nance" (q.v.). Thus, if a person sings a pure
note near a piano it may he observed that the particular string of the piano which of itself gives the same note is set in vibration by the air-waves sent out by the singer.
If air-waves of any length fall upon a soft body, such as a cushion or a curtain, there is absorption, as is shown by the faCt that the re flected waves are much less intense than the incident waves. The energy thus absorbed is not spent in emitting other waves, but is dis sipated throughout the body producing heat effects. Similarly, if ether-waves fall upon an absorbing body. the energy absorbed is dissi pated in general throughout the smallest par ticles of the body producing heat effects. See, however, FLUORESCENCE.