DeviationA ray of light of a single colour incident on an equilateral glass prism is refracted towards the normal N| and travels through the glass at reduced speed. When it emerges into the air it is refracted away from the normal N2 and speeds up again. The ray has been deviated from its original direction; the angle of deviation D is the angle between the original and final directions, as shown in Fig. 17.1. The three prisms in Fig. 17.2(a) have been placed so that they 'deviate' three parallel rays to a single point F. A converging (convex) lens can be thought to consist of a set of differently shaped prisms. In Fig. 17.2(b) rays of light from a point O are deviated by the different sections of the lens to form a real image at 1. This is covered in greater detail in Chapter 18, p.221.
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