Answer
See explanation.
Work Step by Step
a. The energy requirement matches the energy of the maximum-wavelength photon.
$$E= hf=h\frac{c}{\lambda} $$
$$\lambda =\frac{hc}{E}=\frac{1.24\times10^{-6}eV\cdot m}{0.67eV}=1.85\times10^{-6}m$$
b. $$E= hf=h\frac{c}{\lambda} $$
$$\lambda =\frac{hc}{E}=\frac{1.24\times10^{-6}eV\cdot m}{1.12eV}=1.11\times10^{-6}m $$
