Walker, Halliday & Resnick — Electromagnetic Waves: Problem 58

58 The intensity $I$ of light from an isotropic point source is determined as a function of distance $r$ from the source. Figure 33-50 gives intensity $I$ versus the inverse square $r^{-2}$ of that distance. The vertical axis scale is set by $I_s = 400 \text{ W/m}^2$, and the horizontal axis scale is set by $r_s^{-2} = 8.0 \text{ m}^{-2}$. What is the power of the source?