Optical properties of ${\mathrm{In}}_{\mathrm{x}}{\mathrm{Al}}_{1-\mathrm{x}}\mathrm{N}$ are calculated as a function of the concentration of indium and aluminum. Aluminum is partially replaced by indium in an AlN sample, and optical properties of the resulting materials are studied. The fractional concentration of indium is increased gradually from $\mathrm{x}=0$ to $\mathrm{x}=1$ in steps of 0.25. The bandgap decreases with increasing indium concentration, ending up with a narrow gap of $0.9\mathrm{eV}$ for pure InN and a wide gap of $5.4\mathrm{eV}$ for pure AlN. Frequency-dependent reflectivity, absorption coefficient, and optical conductivity of ${\mathrm{In}}_{\mathrm{x}}{\mathrm{Al}}_{1-\mathrm{x}}$ N are calculated and found to be the constituent’s concentration dependent. The maximum value of reflectivity, absorption coefficient, and optical conductivity shifts from higher frequency into the lower frequency region as the material goes from pure AlN to pure InN.