Solution

The average radiation pressure on a perfect mirror for a sinusoidal plane wave incident at an angle$\alpha$(measured with respect to the normal) is:

$P = \frac{2I}{c}\cos^2\alpha$

where I is the average intensity of the incident wave. For a plane electromagnetic wave in vacuum

$I = \dfrac{1}{2}\varepsilon_0 c E_0^2$

where E_0 is the amplitude of the electric field. Substituting and solving for$E_0$:

$P = \varepsilon_0 E_0^2 \cos^2\alpha \quad\Longrightarrow\quad \boxed{E_0 = \frac{1}{\cos\alpha}\sqrt{\frac{P}{\varepsilon_0}}}$

If the angle were measured with respect to the surface,$\cos\alpha$ is replaced by$\sin\alpha$. In the CGS system, the equivalent expression is \displaystyle $E_0 = \frac{\sqrt{4\pi P}}{\cos\alpha}$
(But this is only for knowledge )