$\lambda {\mkern 1mu} {\mkern 1mu}  = {\mkern 1mu} {\mkern 1mu} 4000{\mkern 1mu} \,\mathop A\limits^o {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} \lambda {\mkern 1mu} {\mkern 1mu}  = {\mkern 1mu} {\mkern 1mu} 4000 \times {10^{ - 10}}{\mkern 1mu} m$

$\because \,\,v = \frac{c}{\lambda }\,\,\,\,$

$\therefore \,\,v = \frac{{3 \times {{10}^8}\,m/s}}{{4 \times {{10}^{ - 7}}\,m}}\,\,\, = \,\,0.75 \times {10^{15}}\,{s^{ - 1}}\,\, = \,\,7.5 \times {10^{14}}\,{s^{ - 1}}$

ઉર્જા ની ગણતરી : $E = hv = 6.626 \times  10^{-34} \,J \times  7.5 \times  10^{14}\,s^{-1} = 4.96 \times  10^{-19}\,J$