Page:Elementary Text-book of Physics (Anthony, 1897).djvu/444

430 other things being equal, $$x$$ varies with $$\lambda;$$ hence we must conclude that the greater distance between the bands indicates a greater wave length; that is, that the wave length of red light is greater than that of blue.

351. Measurement of Wave Lengths.—Data may be obtained from any of the above experiments for the determination of the wave length of light. From equation (113) we have $$\lambda = \frac{2bx}{cn},$$ where $$c, b,$$ and $$x$$ are distances that can be measured. The distance $$x$$ is the distance from $$s$$ to a point $$m,$$ the centre of a light band, and $$n$$ equals twice the number of dark bands between $$s$$ and $$m.$$ Better methods than this of measuring wave lengths will be found described in § 355.

352. Interference from Thin Films.—Thin films of transparent substances, such as the wall of a soap-bubble or a film of oil on water, present interference phenomena when seen in a strong light, due to the interference of waves reflected from the two surfaces of the film. Let $$AA, BB$$ (Fig. 133) be the surfaces of a transparent film. Light falling on $$AA$$ is partly reflected and partly transmitted. The reflection at the upper surface takes place with change of sign (§ 132). The light entering the film is partly reflected at the lower surface without change of sign, and returning partly emerges at the upper surface. It is there compounded with the wave at that moment reflected. Let us suppose the light homogeneous, and the thickness of the film such that the time occupied by the light in going through it and returning is the time of one complete vibration. The returning wave will be in the same phase as the one at that moment entering,