## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

(a) $I = 0.099~W/m^2$ (b) $I = 1.6\times 10^6~W/m^2$
(a) The light from the light bulb spreads in all directions from the light bulb. We can find the light intensity on a wall 2.0 meters away. $I = \frac{P}{A}$ $I = \frac{P}{4\pi~R^2}$ $I = \frac{5.0~W}{(4\pi)(2.0~m)^2}$ $I = 0.099~W/m^2$ (b) The light from the laser does not spread out as it leaves the laser. The light stays in a beam with a diameter of 2.0 mm. We can find the light intensity on a wall 2.0 meters away. $I = \frac{P}{A}$ $I = \frac{P}{\pi~R^2}$ $I = \frac{5.0~W}{(\pi)(1.0\times 10^{-3}~m)^2}$ $I = 1.6\times 10^6~W/m^2$