Page:Elementary Text-book of Physics (Anthony, 1897).djvu/460

446 most common effect of such absorption is to generate heat, and there are some surfaces upon which heat will be generated by the absorption of ethereal waves of any length. Langley, by means of the bolometer, has been able to measure the energy throughout the spectrum. He has demonstrated the existence,. in the lunar spectrum, of waves as long as 170,000 tenth metres, or more than twenty-two times as long as the longest that can excite human vision.

363. Intensity of Radiations.—The intensity of radiations can only be determined by their effects. If the radiations fall on a body by which they are completely absorbed and converted into heat, the amount of heat developed in unit time may be taken as the measure of the radiant energy. Let us suppose the radiations to emanate from a point equally in all directions, and represent the total energy in a wave by $$E.$$ Let the point be at the centre of a hollow sphere, of which the radius is $$r,$$ and represent by $$I$$ the energy per unit area of the sphere. Then, since the surface of the sphere equals $$4\pi r^2,$$ we have $$E = 4\pi r^2 I,$$ and That is, the energy which falls upon a given surface is in the inverse ratio of the square of its distance from the source. As we know by experiment that the intensity of light follows the same law, we conclude that the intensity and energy are proportional.

If the surface be not normal to the rays, the radiant energy it receives is less, as will appear from Fig. 139. Let $$ab$$ be a surface the normal to which makes with the ray the angle $$\theta;$$ then $$ab$$ will receive the same quantity of radiant energy as $$a'b',$$ its projection on the plane normal to the ray. But $$a'b'$$ equals $$ab \cos \theta;$$ and if $$I$$ represent the energy on unit area of $$a'b',$$ and $$I'$$