Answer
(a) $E = 0.0100~J$
(b) There are $~~3.2\times 10^{16}~~$ photons in each pulse.
Work Step by Step
(a) We can find the energy in each pulse:
$E = P~t = (0.500~W)(20.0\times 10^{-3}~s) = 0.0100~J$
(b) We can find the energy of each photon:
$E_p = \frac{hc}{\lambda}$
$E_p = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{643\times 10^{-9}~m}$
$E_p = 3.09\times 10^{-19}~J$
We can find the number of photons in each pulse:
$\frac{E}{E_p} = \frac{0.0100~J}{3.09\times 10^{-19}~J} = 3.2\times 10^{16}$
There are $~~3.2\times 10^{16}~~$ photons in each pulse.
