Fringes

Fringes, in the various phenomena of diffraction, mean edgings of colour between light and shade, that may be produced by small screens in the path of light or by small beams of light admitted through orifices in larger screens. The corpuscular theory of light advocated the rectilinear propagation of light corpuscles, and so explained the definite line of demarcation between light and shadow, when any opaque obstacle was placed in the path of a beam of light. It denied the possibility of light travelling round corners in a homogeneous medium, and its supporters showed the apparent invalidity of the wave theory by pointing out that waves, such as those of sound in air or water, will travel round corners. The wave theorists showed by these fringes that light did travel round corners, though not to the same extent as sound-waves by reason of the great difference in the wave-lengths in the two cases. A simple case of formation of diffraction-fringes may be seen when the eyes are directed towards the sun, and so nearly closed that the eyelashes come down in front, and form a sort of grating of fine threads. These act as screens from the light, but, instead of seeing so many black lines of shadow, a series -of bands of colour are seen. The colour exists in those parts in the shadow of the lashes, and is thus separated because of the different powers of the different colour constituents of the sunlight to be deflected round the corners presented by the grating.