## College Physics (4th Edition)

The number of photons incident on the skin is $1.96\times 10^{16}$
We can find the energy of a photon with a wavelength of $300~nm$: $E_p = \frac{hc}{\lambda}$ $E_p = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{300\times 10^{-9}~m}$ $E_p = 6.626\times 10^{-19}~J$ We can find the number of photons incident on the skin if the total energy is $13~mJ$: $\frac{E}{E_p} = \frac{13\times 10^{-3}~J}{6.626\times 10^{-19}~J} = 1.96\times 10^{16}$ The number of photons incident on the skin is $1.96\times 10^{16}$.