General and practical optics / by Lionel Laurance.

  • Laurance, Lionel.
Date:
1914
Catalogue details

Licence: In copyright

Credit: General and practical optics / by Lionel Laurance. Source: Wellcome Collection.

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

    252/390 (page 242)
    CURVATURE SYSTEM The various formulae in connection with mirrors, prisms and lenses may also be deduced from a consideration of the actual paths of the waves them- selves. The folloAving are elementary examples of the application of this method, which is by some writers preferred to the ray theory as repre- senting the actual physical change in shape and direction undergone by the waves in refraction and reflection. Plane Surface.—A B (Fig. 233) is a plane wave front incident obliquely on the surface CD. \i 11^= \ and }x^= 15 the part of the wave which enters at B travels in the same time to F only 2/3 of the distance A E. With B as centre and B Fas radius describe a small arc, a tangent E F from E showing the inclination of the wave front in the dense medium. At the second Fig. 233. Fig. 234. surface a similar construction shows the wave front G H after emergence,, i^'ff being 1-5 times Course of a Wave through a Prism at Minimum Deviation.—Let CBD (Fig. 234) be a prism on which is incident the plane wave AB at angle of incidence i. The portion B of the wave meeting the base of the prism is retarded to a greater extent than A^ the portion in air, so that when the whole wave enters the prism it takes up the position CM, r being the angle of refraction. Since the deviation is supposed to be minimum, the total refraction is symmetrical with respect to the surfaces CB and CD, so that CM bisects the principal angle. The wave is then incident on the second surface at the angle u, and on emergence it is swung over still more towards the base so