Answer
a) 4.4 $\times 10^{18}$ photons
b) 9.4 $\times 10^{18}$ photons
Work Step by Step
(a) We can find the number of photons as follows:
$n=\frac{E_{total}\lambda}{hc}$
We plug in the known values to obtain:
$n=\frac{(2.5J)(350\times 10^{-9}m)}{(6.625\times 10^{-34}J.s)(3\times 10^8m/s)}$
$n=4.4\times 10^{18}photons$
(b) We know that
$n=\frac{E_{total}}{\frac{hc}{\lambda}}$
We plug in the known values to obtain:
$n=\frac{(2.5J)(750\times 10^{-9}m)}{(6.625\times 10^{-34}J.s)(3\times 10^8m/s)}$
$n=9.4\times 10^{18}photons$
