Wavefront
From Biocrawler, the free encyclopedia.
In geometrical optics, a wavefront is the locus (a line or surface in the path of wave motion) of points having the same phase of vibration.
All the points on a have the same phase, since all the molecules on this particular wavefront are at their maximum displacement above the normal level. Likewise t is a wavefront on which all particles are at their maximum displacements below the normal level. Halfway between t and b is another wavefront where the particles are (momentarily) at the normal level, but moving upward.
As the wave spreads out, the wavefronts advance, and in the case of the water wave, the radius of each circular wavefront continuously increases. Huygens' principle tells us how to predict a new wavefront when we know the position of an earlier one. The construction is as follows: Let every point on the wavefront be considered the source of a small wavelet which spreads out in the forward direction. The new wavefront is the envelope of all the wavelets; that is, the line or surface tangent to all the wavelets. If we now consider light waves spreading out in three dimensions, it is evident that the wavelets are small hemispheres, provided the speed of light is the same in all directions; hence we see that a spherical wavefront will remain spherical and that the energy of the wave is carried away equally in all directions. Such directions of energy flow, perpendicular to the wavefronts, are called rays.
Unless specifically stated otherwise, we shall always assume that light travels through a medium equally fast in all directions; such a medium is called an isotropic medium. Glass, water, air, Lucite, and most other common substances are isotropic, but certain transparent crystals have enough structure to give rise to "easy" and "hard" directions for the propagation of light. Such a crystal is said to be anisotropic. In an isotropic medium, the wavelets are spherical, and the rays are always perpendicular to the wavefronts. In an anisotropic medium the wavefronts are ellipsoidal.
Huygens' principle describes the propagation of light in a straight line, since wavelets from a plane wave give rise to a new plane wavefront, and the corresponding rays are parallel to each other. Such a plane wavefront is a good model for a section of a very large spherical wavefront; for instance, sunlight strikes the earth in wavefronts of radius 93 million miles. For all practical purposes, such a wavefront is "plane" and the rays are "parallel" to each other.
Newton never accepted the wave model of light, for he felt (quite justifiably) that if light waves existed they would naturally "bend around corners" and would not travel in straight lines through openings or around obstacles, as they are observed to do. Newton did not realize that the extreme smallness of the wavelength of light compared with the size of the usual opening or obstacle could account for this behavior.
