Page:Elementary Text-book of Physics (Anthony, 1897).djvu/443

§ 350] The phenomena described above may be obtained experimentally in several ways. Young admitted sunlight into a darkened room through a small hole in a window-shutter. It fell upon a screen in which were two small holes close together, and, on passing through these, was received upon a second screen. Light and dark bands were observed upon this screen, the distances of which from the central band were in accordance with theory.

Fresnel received the light from a small luminous source upon two mirrors making a very large angle, as in Fig. 120. The light reflected from each mirror proceeded as though from the image of the source produced by that mirror. The reflected light, therefore, consisted of two wave systems, from two precisely similar sources $$A$$ and $$B.$$ Light and dark bands were formed in accordance with theory. In order that the experiment may be successfully repeated reflection must take place from the front surface of each mirror only the angle made by the mirrors must be nearly 180°, and the reflecting surfaces must meet exactly at the vertex of the angle. Two similar sources of light may be obtained also by sending the light through a double prism, as shown in Fig. 131. Light from $$A$$ proceeds after passing through the prism as from the two virtual images $$a$$ and $$a'.$$

A divided lens, Fig. 122, serves the same purpose. The light from $$A$$ is concentrated in two real images $$a$$ and $$a',$$ from which proceed two wave systems as in the previous cases. What are really seen in these cases, when the source of light is white, are iris-colored bands instead of bands of light and darkness merely. When the light is monochromatic, the bands are simply alternations of light and darkness, the distances between them being greatest for red light, and least for blue. From equation (113) it appears that,