Given: Energy gap, $E_g = 1.9 \text{ eV}$ We know that the wavelength $\lambda$ of the emitted light is related to the energy gap by the equation: $E_g = \frac{hc}{\lambda}$ $\lambda = \frac{hc}{E_g}$ Substituting the standard values ($hc \approx 1240 \text{ eV}\cdot\text{nm}$ or more precisely $1242 \text{ eV}\cdot\text{nm}$): $\lambda = \frac{1242 \text{ eV}\cdot\text{nm}}{1.9 \text{ eV}} \approx 653.68 \text{ nm} \approx 654 \text{ nm}$

Alternatively, using standard SI units: $h = 6.63 \times 10^{-34} \text{ J s}$ $c = 3 \times 10^8 \text{ m/s}$ $E_g = 1.9 \text{ eV} = 1.9 \times 1.6 \times 10^{-19} \text{ J} = 3.04 \times 10^{-19} \text{ J}$ $\lambda = \frac{6.63 \times 10^{-34} \times 3 \times 10^8}{3.04 \times 10^{-19}}$ $\lambda = \frac{19.89 \times 10^{-26}}{3.04 \times 10^{-19}} \text{ m} = 6.54 \times 10^{-7} \text{ m}$ $\lambda = 654 \times 10^{-9} \text{ m} = 654 \text{ nm}$