Answer
The wavelengths $\lambda$ are less than or equal to $3.5\times10^{-7}m$.
Work Step by Step
The minimum-frequency (i.e., maximum-wavelength) photon that enables conduction has an energy equal to the energy gap.
$$E_g= hf=\frac{hc}{\lambda}$$
$$\lambda = \frac{hc}{E_g}$$
$$=\frac{(6.63\times10^{-34}J \cdot s)(3.00\times10^8m/s)}{(1.60\times10^{-19}J/eV)(3.6eV)}$$
$$=3.5\times10^{-7}m $$
This is the maximum wavelength that will excite the electron, so the range of wavelengths that work are those less than (or equal to) that.