Answer
a. 1.12 eV.
b. See explanation.
Work Step by Step
a. The energy gap matches the energy of the maximum-wavelength photon.
$$E_{gap}= hf=h\frac{c}{\lambda} $$
$$E =\frac{1.24\times10^{-6}eV\cdot m}{1.11\times10^{-6}m}=1.12 eV$$
b. Visible-light photons have enough energy to raise electrons from the valence to the conduction band in silicon, which means that visible light photons will be absorbed. This makes silicon opaque to visible light.
