Page:Elementary Text-book of Physics (Anthony, 1897).djvu/445

§352] and, therefore, opposite in phase to the wave then reflected. The reflected and emerging waves destroy each other, or would do so if their amplitudes were equal, and the result is that, apparently, no light is reflected. If the light falling on the film be white light, any one of its constituents will be suppressed when the time occupied in going through the film and returning is the period of one vibration, or any whole number of such periods, of that constituent. The remaining constituents produce a tint which is the apparent color of the film. Similar phenomena are produced by the interference of that portion of the incident light which is transmitted directly through the film, with that portion which is transmitted after undergoing an even number of internal reflections. Since these reflections occur without change of sign, the thickness of the film for which the reflected light is a minimum is that for which the transmitted light is a maximum.

This theory must be slightly modified on account of the internal reflections in the film. The light which enters the film and is reflected does not all pass out in the reflected beam, but part of it is again sent through the film to the other surface, when it is again divided, so that the reflected and transmitted beams both contain light that has been several times reflected. The theory shows that the reflected beam is totally extinguished when the thickness is that indicated by the elementary theory, and that the transmitted beam is never totally extinguished, but merely passes through a minimum intensity. This conclusion is confirmed by observation.

Newton was the first to study these phenomena. He placed a plane glass plate upon a convex lens of long radius, and thus formed between the two a film of air, the thickness of which at any point could be determined when the radius of the sphere and the distance from the