Total internal reflection in prisms
The problem of providing an untarnishable mirror which gives one image only has been solved in the submarine periscope by using 45° right-angled glass prisms, as shown in Fig. 23.16 (a). Actually a submarine periscope is a combined periscope and
telescope, but for simplicity in the diagram the lenses have been omitted. Light enters the faces of the prisms normally and falls on the hypotenuse face internally at an angle of incidence of 45°. Total reflection occurs here, since the critical angle for ordinary glass is about 42° (page 255).
For reasons explained in chapter 24, it is necessary to place slides or films upside down in a projector in order to obtain a picture the right way up on the screen. It is not always possible to do this, since it is occasionally necessary to project an image of a thin glass cell containing a liquid. For example, polarization on the copper plate
of a simple cell (see page 397) may be shown on a screen by this method. The image is obtained the correct way up by placing an erecting prism in front of the projection lens. Fig. 23.16 (b) shows how a right-angled prism is used for this purpose. Light enters the face of the prism approximately parallel to the base or hypotenuse face. Total reflection occurs at the base, with the result that the rays passing through are inverted.
A third example of employing total internal reflection in right-angled prisms is found in prism binoculars. A prism binocular is simply a pair of telescopes conveniently shortened in length by causing the light to traverse the tube three times instead of once.
Iflight is incident erpendicular to the ypotenuse face of the prism it will undergo two internal reflections, finally emerging arallel to its original path but travelling in he opposite direction. Fig. 23.17 ndicates how two prisms are used in this manner in the binocular. Note that the rays A and B are inverted, iving the added advantage that the final image is the right way up (see also page 72)