Radiation pressure at a oblique surface

Question

I am doing a problem that asks the radiation pressure of a beam of light on a oblique surface. The problem says: A laser beam of intensity I reflects from a flat, totally reflecting surface of area A, with a normal at angle theta with the beam. Write an expression for the beam’s radiation pressure.".

Would someone explain to me where the expression "I/c²" comes from in this solution?

Answer 1

The photon momentum is $p = \frac{h}{\lambda} = \frac{E}{c} = \frac{u \cdot \Delta V}{c}$ with $u$ being the energy density and $\Delta V$ is the volume given.

And the Intensity $I$ is the energy density multiplied by the velocity, or vice versa $u = \frac{I}{c}$ in this case. Combining both gives $p = \frac{I}{c^2}\cdot \Delta V$.