[填空题]
The energy gap between valence and conducycmk - sa7e9 n,x7x0sction bands in germjg fowm x 2;1pe/n08utanium is 0.72 eV. What range of wavelengths can a photon have to o;0tfgmw nx p28 1eju/excite an electron from the top of the valence band into the conduction band? $\lambda \leq$ $ \ \mu\text{m}$
