c
અહી, \( h = 6.6× 10^{-34} Js, c = 3 × 10^ 8 m s^{-1} ; 1 eV = 1.6 × 10^{-19} J\)

બેન્ડગેપ ઉર્જા \({E_g} = \frac{{hc}}{\lambda }\)

\(\therefore \lambda = \frac{{hc}}{{{E_g}}} = \frac{{6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}{{57 \times {{10}^{ - 3}} \times 1.6 \times {{10}^{ - 19}}}} = 0.21710 \times {10^{ - 4}} m = 217100 \times {10^{ - 10}} m\)

\( = 217100 \mathop A\limits^ \circ (\because {10^{ - 10}} m = 1\mathring A\)