less than the critical angle escape.This area through which light can escape forms a circle of radius r, as illustrated in Fig 13.13.
Light radiating from the observer and light entering the observer’s eye follow the same path. It should therefore be clear that only light entering the water from within the circle of radius r can be observed by the diver.As a result, the region around this circle is in darkness.
c More examples Fig 13.13: Illustration of Snell’s window
Other examples of the use of total internal reflection include periscopes, prism binoc- ulars, reflectors on bicycles or road signs. One of the main advantages of total internal reflection over plane mirrors is that all the light is reflected; none is absorbed. In the best ordinary mirror available, about 4 per cent of the light is absorbed.
EXERCISE 13.2TOTAL INTERNAL REFLECTION
Q1 Calculate the critical angle for quartz, given that it has a refractive index of 1.55.
Q2 What is the refractive index of ice, given that it has a critical angle of 49.81°? Q3 A diver looks up at the surface of a still lake from a depth of 15 m and sees a disc of light, known as Snell’s window (Fig 13.14). Calculate the radius of the
disc, given that the refractive index of water is .4 3
Fig 13.14 Lenses
Lenses work on the principle that light is refracted twice—once when it passes from air to glass and once again when it passes from glass to air.
Optic centre (C) Focus (F) Principal axis Focus 15 m r
Optic centre (C)
Converging lens (Convex lens)
Diverging lens (Concave lens)
Focus (F) Principal axis Fig 13.15:Terms used for lenses
TERMS USED TO DESCRIBE LENSES The optic centre is the centre point of the lens. The principal axis is the straight line passing through the optic centre that enters the lens at right angles to the face of the lens.
The principal focus or simply the focus is the point through which light travelling parallel to the principal axis converges to for a convex lens or appears to diverge from for a concave lens. The focal length is distance from the focus to the optic centre.
