## College Physics (4th Edition)

(a) $E = 0.0100~J$ (b) There are $~~3.2\times 10^{16}~~$ photons in each pulse.
(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.