I was recently taught that radiation pressure $P$ is given by $\frac{I}{c}$ in a vacuum.
To derive this I wrote that from the fact that $I=\frac{Fv}{A}=> P=\frac{I}{c}$
However, according to classical physics, light is massless so how do I explain this using classical theory?
Here $I$ is intensity, $c=v$ = the speed of light, and A is the area of an arbitrary surface normal to the source.
