Answer
The wavelength must be less than 1.7 micrometers.
Work Step by Step
The minimum-frequency (i.e., maximum-wavelength) photon that enables conduction has an energy equal to the energy gap.
$$E_g= hf=\frac{hc}{\lambda}$$
$$\lambda = \frac{hc}{E_g}$$
$$=\frac{(6.63\times10^{-34}J \cdot s)(3.00\times10^8m/s)}{(1.60\times10^{-19}J/eV)(0.72eV)}$$
$$=1.7\times10^{-6}m $$
This is the maximum wavelength that will excite the electron into the conduction band, so the range of wavelengths that work are those less than 1.7 micrometers.
