The principles and methods of geometrical optics : especially as applied to the theory of optical instruments / by James P. C. Southall.

  • Southall, James P. C. (James Powell Cocke), 1871-1962.
Date:
1913
Catalogue details

Licence: In copyright

Credit: The principles and methods of geometrical optics : especially as applied to the theory of optical instruments / by James P. C. Southall. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    Fig. 1. Huygens's Construction of the Wave- Front. tropic medium that are in this particular initial phase of vibration, and which is the Wave-Front at this instant. According to Huygens, every point P in the wave-front, from the instant that the disturb- ance reaches it, will become a new source or centre of disturbance, from which secondary waves will be propagated in all directions. More- over, Huygens assumed that these secondary waves, originating at all the points affected by the principal wave, interfere with each other in such fashion that their resultant sensible effects are produced only at the points of the surface which envelops at any given instant all the secondary wave-fronts, and that this enveloping surface is, therefore, the wave-front at that instant. Obviously, in an isotropic medium, such as is here supposed, this sur- face will be a sphere described around 0 as centre. Accordingly, if waves diverge from a luminous point 0 (Fig. 2), and if an opaque plane screen HJ with an opening A B is interposed in front of the advanc- ing waves, the wave-front at any time t may be constructed as follows: Consider all the points, such as M, which lie in the plane of the screen at the place where the aperture is made. As soon as the disturbance arrives at one of these points, it will become a new centre of disturbance, from which will diverge, therefore, secondary spherical waves. In general, the radii of these secondary waves will be different. Thus, in the diagram, as here drawn, the point des- ignated by A is nearer the source 0 than the point designated by M, so that the disturbance must arrive at A first, and hence the secondary Huygens's Construction of the Wave-Front. Spherical waves di- verging- from the point-source O and passing through an opening AB in the opaque screen HJ. The arc DC is a section of the portion of the spher- ical wave-front <r which contains the points beyond the screen which have been reached by the disturbance.