Answer
The wavelength is 0.96 micrometers.
Work Step by Step
The wavelength of the photon that is emitted has an energy equal to the energy gap.
$$E_g= hf=\frac{hc}{\lambda}$$
$$\lambda = \frac{hc}{E_g}$$
$$=\frac{(6.63\times10^{-34}J \cdot s)(3.00\times10^8m/s)}{(1.60\times10^{-19}J/eV)(1.3eV)}$$
$$=9.6\times10^{-7}m $$
