The mirage Mirages are usually associated with hot deserts. The traveller in a desert often sees what appears to be a sheet of water a short distance ahead of him. This he is never able to reach, since it is an optical illusion. It is not necessary to make a journey to the Sahara in order to […]
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. […]
Multiple images formed by a thick glass mirror Ordinary mirrors, made by silvering the back face of a sheet of glass, suffer from the disadvantage that extra images are formed by reflection from the front surface of the glass and also by multiple internal reflection inside the glass. Fig. 23.15 shows how the images are formed. Ordinarily […]
To measure the critical angle and refractive index of the material of a prism An equilateral glass prism ABC is placed on a sheet of paper and its outline drawn on the paper with the aid of a ruler in a manner similar to that described for the glass block on page 249. An object pin 0 is then […]
The fish’s-eye view At whatever depth a fish happens to be it has a full view of everything above the water, provided, of course, that the water surface is unruffled. Fig. 23.13 explains this and shows how the fish enjoys a 1800 field of view apparently all squeezed into a cone of angle about 980 (i.e., twice the […]
Relation between critical angle and refractive index For crown glass of refractive index n = 1.5, the critical angle c is about 42°. In the case of water (n = 1.33) it is about 49°. These values are obtained from the above equation, using a table of sines.
Total internal reflection. Critical angle When light passes from one medium to a more optically dense medium there will always be both reflection and refraction for all angles of incidence (Fig. 23.11). But this is not always so when light passes from one medium to a less optically dense medium. Suppose we consider a ray passing from glass […]
To measure refractive index by the real and apparent depth method (a) Glass. A glass block is placed vertically over a straight line ruled on a sheet of paper (Fig. 23.10 (a)). A pin on a sliding cork adjacent to the block is then moved up or down until there is no parallax between it and the […]
Refractive index related to real and apparent depth The way refractive index is related to real and apparent depth is illustrated in Fig. 23.9. Once again the horizontal scale is exaggerated, but it is to be understood that OBC is a ray very close to the normal which enters the eye from a point 0 at the […]
Real and apparent depth A thick slab of glass appears to be about two-thirds of its real thickness when viewed from vertically above. Similarly, water in a pond appears to be only three-quarters of its true depth. Fig. 23.8 shows how this comes about. Rays from a point 0 at the bottom of the slab are refracted away from […]