## College Physics (4th Edition)

The equation $~~I = ~\lt u \gt c~~$ is dimensionally consistent.
$I$ is intensity in units of $W~m^{-2}$ $\lt u \gt$ is energy density in units of $J~m^{-3}$ $c$ is the speed of light in units of $m~s^{-1}$ We can verify the units of the term $\lt u\gt ~c$: $(J~m^{-3})~(m~s^{-1}) = J~s^{-1}~m^{-2} = W~m^{-2}$ We can see that both sides of the equation $~~I = \lt u \gt c~~$ have units of $\frac{W}{m^2}$ Therefore the equation $~~I = ~\lt u \gt c~~$ is dimensionally consistent.