The intensity I of light from an isotropic point source is d

The intensity I of light from an isotropic point source is determined as a function of distance r from the source. The figure gives intensity I versus the inverse square r6-2 of that distance. The vertical axis scale is set by I_s = 217 W/m^2, and the horizontal axis scale is set by r_s^-2 = 8.8 m^-2. What is the power of the source?

Solution

I_s = 217 W/m ²

r_s^-2 = 8.8 m ^-2

extreme value in horizontal axis = (5/4) 8.8

                                              = 11 m ^{- 2}

Area under line and horizontal axis = I_s (11 m ^-2)

                                                   = (217 W/m ²)(11 m^-2)

                                                   = 2387 W

                                                   = 2.387 kW