1. Green light of frequency 5.7 × 10¹⁴ Hz is travelling in air at a speed of 3.0 × 10⁸ m/s. The light is incident on the surface of a transparent solid.

Fig. 6.1 shows the wavefronts and the direction of travel of the light in the air.        

The light travels more slowly in the transparent solid.

Explain, in terms of the wavefronts, why the light changes direction as it enters the solid. You may draw on Fig. 6.1 as part of your answer.