Physics - Modern Physics Question with Solution | TestHub

The energy of light of wavelength $\lambda$ is given by $$\mathrm{E}=\mathrm{hv}=\frac{\mathrm{hc}}{\lambda }\Rightarrow \lambda =\frac{\mathrm{hc}}{\mathrm{E}}$$ Here, $\mathrm{h}=$ Planck's constant $=6.63\times 10^{-34}\mathrm{J}-\mathrm{s}$ $\mathrm{c}=$ speed of light $=3\times 10^8\mathrm{m}/\mathrm{s}$ $\mathrm{E}=$ energy gap $=1.9\mathrm{eV}=1.9\times 1.6\times 10^{-19}\mathrm{J}$ Substituting the given values in Eq. (i), we get $$\begin{array}{rl}\Rightarrow \lambda & =\frac{6.63\times 10^{-34}\times 3\times 10^8}{1.9\times 1.6\times 10^{-19}}\\ & =6.54\times 10^{-7}\mathrm{m}\approx 654\mathrm{nm}\end{array}$$ Thus, the wavelength of light emitted from LED will be $654\mathrm{nm}$.