(a) The energy-band gap in germanium is 0.72 eV. What wavelength range of visible light will be transmitted by a germanium crystal? (Think about it carefully!)
(b) Now consider a crystal of an insulator whose energy band gap is 3.6 eV. What wavelength range of visible light will this crystal transmit? 

(c) Justify each of your answers to (a) and (b).