## College Physics (4th Edition)

(a) The intensity of the light just outside the laser is $7.96\times 10^5~W/m^2$ (b) The intensity of the light where it hits the surface of the moon is $1.76\times 10^{-9}~W/m^2$
(a) We can find the intensity of the light just outside the laser: $I = \frac{P}{A}$ $I = \frac{P}{\pi~r^2}$ $I = \frac{10~W}{(\pi)(2.0\times 10^{-3}~m)^2}$ $I = 7.96\times 10^5~W/m^2$ The intensity of the light just outside the laser is $7.96\times 10^5~W/m^2$ (b) We can find the intensity of the light where it hits the surface of the moon: $I = \frac{P}{A}$ $I = \frac{P}{\pi~r^2}$ $I = \frac{10~W}{(\pi)(4.25\times 10^4~m)^2}$ $I = 1.76\times 10^{-9}~W/m^2$ The intensity of the light where it hits the surface of the moon is $1.76\times 10^{-9}~W/m^2$