Page:The American Cyclopædia (1879) Volume X.djvu/449

LIGHT 443 aperture. Pass the homogeneous beam through a convex lens of short focal distance, converg- ing the beam to a physical point at a, and place an opaque screen 5, having a sharp edge, be- yond the focus to intercept the lower portion of the luminous cone, allowing the upper por- tion to be projected upon a second screen, still further removed. Below the right line c d the second screen will not be entirely in shadow, but will be faintly illuminated for a short distance, the light gradually passing into obscurity. That portion of the cone which is above the edge of the first screen and which falls upon the second will not be uniformly il- luminated, as might be supposed ; but there will be an alternate series of light and dark bands, proceeding from the intersection of the line c dj and extending upward until they gradually disappear. The light and dark bands are not clearly separated, but their edges are more or less blended. If, instead of red, violet light is employed, both dark and light bands, as well as the whole spectrum, will be nar- rower. Careful observation will also discover that the fringes are not straight lines, but are sensibly curved, the concavity of the curvature being turned downward toward the shadow. By using an eyepiece with a micrometer, Fres- nel accurately determined the distances of the bands from the shadows at different points, and found these curves to be hyperbolas. If in this experiment the light proceeds from a source which has any considerable cross section, in- stead of from a point or a very minute orifice, each line in it parallel to the first screen will tend to produce a different system of fringes, the dark band of one coinciding with the light band of the next, and thus the phenomena of diffraction will not be produced. If, instead of employing an edge, the light is allowed to pass the opposite edges of a very narrow opaque body, as a hair or a fine wire, the phenomena observed by Grimaldi will appear ; but the use of monochromatic light will render them more distinct. On each side of the geometrical pro- jection there will appear a set of parallel bands or fringes, like those produced without the geometrical projection by the single edge ; but within' it, instead of the gradually shaded light, there will also be a series of dark and light bands similar to those outside, only narrower and more clearly marked. These are called the interior fringes, and they also have the form of hyperbolas, but of less curvature. Newton explained the phenomena of diffrac- tion by supposing that the rays of light in pass- ing the edges of bodies are inflected, in con- sequence of the attraction or repulsion between their particles and the matter composing the edges of the bodies so passed ; an explanation similar to that which he offered for reflection and refraction. He supposed that the particles of light in passing the edge of an opaque body are repelled when they arrive at a certain point ; and therefore that those passing nearest the edge are inflected the most, and so made to intersect others less inflected, thus produ- cing a caustic (see OPTICS), within which no rays will pass, and which forms the boundary of the visible shadow. He explained the ap- pearance of the fringes by supposing the attrac- tive and repulsive forces between the particles of each ray and the matter of the edge of the body to alternate, and thus throw them into a serpentine course, the intersections of the rays producing a series of caustics which dimin- ish in intensity from the edge of the shadow. To explain the appearance of the prismatic colors, it was only necessary to apply his theo- ry of the decomposition of white light, which supposes that the various colored rays are unequally attracted by the refracting body through which they pass. That the explana- tions of Newton are insufficient, aside from other evidence, appears from a consideration of the fact that no difference in the degree of diffraction is produced by increasing the densi- ty and therefore the attraction of the matter composing the diffracting edge. The supposi- tion of Mairan and Du Tour that diffraction is caused by refraction of the rays of light in passing through a prismatic film of air con- densed by the inflecting body is equally open to objection, for these atmospheric prisms would necessarily vary in form with the nature and density of the body, and would conse- quently possess different refractive powers. To Young and Fresnel we owe the explana- tion of the phenomena of diffraction according to the principle of the interference of waves. The formation of the exterior fringes was ascribed by Young to the interference of two portions of light, one of which passes by the body and is slightly deflected, while the other is thrown further out of its course by being obliquely reflected by the edge. The pro- duction of the inferior fringes he ascribed to the bending of the waves inward, and the consequent interference which takes place between the rays that intersect each other from the opposite sides. That this latter explanation is correct is shown by the fact that the bands do not appear unless the ob- struction is sufficiently narrow to allow rays coming from the opposite sides of the body to intersect each other ; when the body is not nar- row, the same effect is produced on either side as when -a single edge is employed. The ex- planation of the formation of the exterior fringes, however, does not agree with certain observed facts. Fresnel allowed the rays of light to pass both the back and the edge of a razor, and produced fringes which were alike in breadth and intensity ; which could not have been the case if the interference were caused by the reflection of a portion of light by the body, because then there would be a greater condensation by the back than by the edge. Fresnel ascribed the effects to partial or sec- ondary waves, which are separated from the principal wave by meeting an obstacle, and are subdivided into an indefinite number of equal