Geometrical optics / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1913
Catalogue details

Licence: In copyright

Credit: Geometrical optics / by Archibald Stanley Percival. 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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    spectrometer, and from these data the value of can be found by this formula. The prisms that are prescribed for spectacles are usually- very weak, i.e. the angle A is rarely more than 5°, and for these a simple approximate formula can be given for the amount of deviation they induce. When \fr and if/' are both very small, sin if/ and sin ^ may be replaced by ^ and if/, and the formula for the position of minimum deviation D = 2i^ — A may be replaced by 2/n// — A or (jll — 1) A, since A = 2\ff'. This is a most useful formula if only used for weak prisms when placed in the symmetrical position; thus, as the value of fx for glass is little more than 1*5, we find that the deviation produced by such a weak prism is about half its apical angle. Image by Refraction through a Prism.—We have so far been only considering the course of light refracted by a prism; when we try to form some idea of the appearance of an object when viewed through a prism, we meet with several difficulties that will only be briefly mentioned here. We have seen that the deviation is not proportional to the angle of incidence, and as an object of appreciable size presents many points from each of which light is falling on the prism at a different angle, it will be seen that the matter is a very complicated one. In Fig. 22 an object Vp is supposed to be viewed by an eye in the neighbourhood of ET, the narrow pencil from P that enters the eye being represented in the position of minimum deviation. Clearly a virtual image of P will be formed at Q, where the prolongations of the refracted rays intersect. It is obvious that the image will be displaced towards the edge of the prism, and that it will seem nearer by one-third the thickness of the prism traversed (p. 40). Now, if we consider light emerging from p, it is clear that if it fell on the prism in the position of minimum deviation it would form a virtual image in the neighbourhood of q. But an eye at K could not receive this pencil; the only light that would enter it must have had some such initial direction as px, and the final image of p would be indistinct, for the