= Solution
For a sinusoidal <plane electromagnetic wave> in <vacuum>, write the maximum <electric field> as $E_0$. The <magnetic field> maximum is $B_0=E_0/c$, so the cycle-averaged <Poynting vector> magnitude, or <light intensity>, is
$$
I=\frac{E_0B_0}{\mu_0}\langle\cos^2\omega t\rangle
=\frac12\epsilon_0cE_0^2,\qquad
\boxed{E_0=\sqrt{\frac{2I}{\epsilon_0c}}.}
$$
Each <photon> carries <energy> $hf$, where $h$ is the <Planck constant>. Therefore the <photon number flux density>, namely the number per unit normal <area> per unit time, is
$$
\boxed{\Phi_N=\frac I{hf}=\frac{\epsilon_0cE_0^2}{2hf}.}
$$
Multiplying by an illuminated normal <area> gives the <photon> arrival rate. Each absorbed <photon> transfers <momentum> $hf/c$ at <normal incidence>, giving the <radiation pressure on a perfectly absorbing surface>
$$
\boxed{p=\frac Ic=\frac12\epsilon_0E_0^2.}
$$
These are time-averaged quantities. At oblique <angle of incidence> $\theta$, the normal <radiation pressure> is $I\cos^2\theta/c$; the simple displayed expression assumes <normal incidence>.
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