Question

The power output of a laser is measured in units of watts (W), where one watt is equal to 1 one
joule per second (1 W = 1 J.s-1) What is the number of photons emitted per second by a 1.00 mW nitrogen laser?
The wavelength emitted by a nitrogen laser is 337 nm.

Answer 1

energy for 1 photons:

$E=h\dfrac{c}{\lambda}$

1.00 mW = 1 x 10⁻³ J/s

so number of photons emitted:

$n=\dfrac{1\times 10^{-3}}{h\dfrac{c}{\lambda} }\\\\\tt h=Planck's\:constant(6.626.10^{-34} Js)\\\\\tt c=speed\:of\:light=3.10^8m/s\\\\\lambda=wavelength(m)$

$\tt n=\dfrac{1\times 10^{-3}\times 337\times 10^{-9}}{6.626\times 10^{-34}\times 3\times 10^8}\\\\\boxed{\bold{n=1.695\times 10^{15}}}$