Photographs of such patterns appear in Fig. 9 in the directions parallel to the two sides. Thus for a rectangular or square aperture, the wavefront may be subdivided into elements parallel to either of two adjacent sides, giving an intensity distribution which follows the curve of Fig. The main features of Fraunhofer diffraction patterns of other shapes can be understood with the aid of the vibration curve. The slit would have to be much narrower than this, or the wavelength much longer, for the approximation to cease to be valid. ( 3) gives the angle as only 0.0005 radian, or 1.72 minutes of arc. For a slit 1 mm wide, for example, and green light of wavelength 5 × 10 −5 cm, Eq. In this way, the distribution of the intensity of light in any Fraunhofer diffraction pattern may be determined. The intensity on the screen is then proportional to the square of this resultant amplitude. When the individual vectors represent the contributions from infinitesimal surface elements (as they must for the Huygens wavelets), the diagram becomes a smooth curve, the vibration curve, shown in Fig. The resultant of all elements would be the vector A. They will, however, generally differ in phase, so that if the elements were small but finite each would be drawn at a small angle with the preceding one, as shown in Fig. If these elements are assumed to be of equal area, the magnitudes of the amplitudes to be added will all be equal.
The vibration curve results from the addition of a large (really infinite) number of infinitesimal vectors, each representing the contribution of the Huygens secondary waves from an element of surface of the wavefront. All of these may be demonstrated especially well by using the light beam from a neon-helium laser. Photographs of some diffraction patterns of each class are shown in Figs. An alternative way of distinguishing the two classes, therefore, is to say that Fraunhofer diffraction concerns the effects near the focal point of a lens or mirror, while Fresnel diffraction concerns those effects near the edges of shadow. The diffraction effects occur chiefly near the borders of the geometrical shadow, indicated by the broken lines. In Fresnel diffraction, no lenses intervene. A second lens L 2 focuses parallel diffracted beams on the observing screen F, situated in the principal focal plane of L 2. In Fraunhofer diffraction, the source lies at the principal focus of a lens L 1, which renders the light parallel as it falls on the aperture. The light originates at a very small source O, which can conveniently be a pinhole illuminated by sunlight. 1 shows the experimental arrangements required to observe them for a circular hole in a screen s. To illustrate the difference between methods of observation of the two types of diffraction, Fig. At that time, it played an important part in establishing the wave theory of light. A complete explanation of Fresnel diffraction has challenged the most able physicists, although a satisfactory approximate account of its main features was given by A. The latter class includes the effects in divergent light, and is the simplest to observe experimentally. The former concerns beams of parallel light, and is distinguished by the simplicity of the mathematical treatment required and also by its practical importance. There are two main classes of diffraction, which are known as Fraunhofer diffraction and Fresnel diffraction.
See also: Radio-wave propagation X-ray diffraction The effects for light are important in connection with the resolving power of optical instruments. For discussion of the phenomenon as encountered in other types of waves See also: Electromagnetic wave Electron diffraction Neutron diffraction Soundĭiffraction is a phenomenon of all electromagnetic radiation, including radio waves microwaves infrared, visible, and ultraviolet light and x-rays. Some important differences that occur with microwaves will also be mentioned. Although diffraction is an effect exhibited by all types of wave motion, this article will deal only with electromagnetic waves, especially those of visible light. Most diffraction gratings cause a periodic modulation of the phase across the wavefront rather than a modulation of the amplitude. More exactly, diffraction refers to any redistribution in space of the intensity of waves that results from the presence of an object that causes variations of either the amplitude or phase of the waves. The bending of light, or other waves, into the region of the geometrical shadow of an obstacle.
0 kommentar(er)
