= Radiation pressure
{title2=$p_{\rm rad}$}
{wiki}
<Radiation pressure> is the normal <momentum flux> transferred by <electromagnetic radiation> to a surface. A <photon> travelling in <vacuum> carries <momentum> $p_\gamma=E_\gamma/c=h\nu/c$, where $E_\gamma$ is its <energy>, $\nu$ its <frequency>, $h$ the <Planck constant> and $c$ the <speed of light>. <Photon absorption> transfers that <momentum>; <specular reflection> reverses its normal component. This lets <light> exert a <force> even when almost no <energy> becomes <heat> in the receiver.
Two useful formulas describe different preparations. A directed beam normally incident on a stationary perfect absorber gives <pressure> $I/c$, and on a stationary perfect mirror gives $2I/c$. Here $I$ is <light intensity>, the incident <power> per beam cross-sectional <area> perpendicular to propagation, in $\mathrm{W\,m^{-2}}$. An <isotropic> <photon gas> has <pressure> $u/3$, where $u=U/V$ is its radiation <internal energy> per <volume (thermodynamics)>, in $\mathrm{J\,m^{-3}}$. Stored <energy density> and travelling <energy flux> must be distinguished: an <isotropic> field can have zero net <energy flux> and positive <radiation pressure>.
The physical appeal is that the same <momentum conservation> connects a single <photon> collision, a mirror's measurable <force>, and a radiation-filled cavity's <equation of state>. The directional calculation below complements <radiation pressure on a perfectly absorbing surface>; the equilibrium spectrum and its <temperature> dependence belong to <blackbody radiation> and the <photon gas>.
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