11. An LED is constructed from a \( p - n \) junction based on a certain compound semiconductor whose energy gap is \( 2.0\mathrm{eV} \). What is the wavelength of the emitted light? (10%) [Note: electron charge, \( e = 1.6\times 10^{-19}\mathrm{J / eV} \). Planck constant, \( h = 6.6\times 10^{-34}\mathrm{J}\cdot \mathrm{sec} \). Speed of light, \( c = 3\times 10^{8}\mathrm{m / sec} \)]