(a)[4]
White light is sent through a cloud of cool low-pressure gas, as shown in Fig. 10.1. In the light that emerges from the gas, the continuous spectrum is seen to contain several darker lines. Use the idea of discrete electron energy levels to explain why these darker lines appear.
(b)[5]
The highest electron energy bands in a solid are shown in Fig. 10.2. Use band theory to explain how the resistance of a light-dependent resistor (LDR) depends on light intensity.