# A certain laser transition emits $6.37\times10^{15}$ quants per second per square meter. Calculate the power out put in joule per square metre per second. Given $\lambda$ = 632.8 nm.

$(a)2\times10^{-3}J/m^2/sec\qquad(b)1\times10^{-2}J/m^2/sec\qquad(c)2\times10J/m^2/sec\qquad(d)1\times10J/m^2/sec$

Energy falling per square meter per second = No. of quanta falling/$m^2/sec\times$ energy of one quanta
$= 6.37\times10^{15}\times\large\frac{hc}{\lambda}$
$=6.37\times10^{15}\times\large\frac{6.625\times10^{-34}\times3.0\times10^8}{632.8\times10^{-9}}$
$=2\times10^{-3} J/m^2/sec$