Editing Radiation pressure (section)

=== Radiation pressure in terms of photons ===
{{See also|Photons|Momentum}}
Electromagnetic radiation can be [[wave–particle duality|viewed]] in terms of particles rather than waves; these particles are known as [[photons]]. Photons do not have a rest-mass; however, photons are never at rest (they move at the speed of light) and acquire a momentum nonetheless which is given by:
<math display="block">
p = \dfrac{h}{\lambda} = \frac{E_p}{c},
</math>
where {{math|''p''}} is momentum, {{math|''h''}} is the [[Planck constant]], {{math|''λ''}} is [[wavelength]], and {{math|''c''}} is speed of light in vacuum. And {{math|''E<sub>p</sub>''}} is the energy of a single photon given by: 
<math display="block">
E_p = h \nu = \frac{h c}{\lambda}
</math>

The radiation pressure again can be seen as the transfer of each photon's momentum to the opaque surface, plus the momentum due to a (possible) recoil photon for a (partially) reflecting surface. Since an incident wave of irradiance {{math|''I<sub>f</sub>''}} over an area {{math|''A''}} has a power of  {{math|''I<sub>f</sub>A''}}, this implies a flux of {{math|''I<sub>f</sub>''/''E<sub>p</sub>''}} photons per second per unit area striking the surface. Combining this with the above expression for the momentum of a single photon, results in the same relationships between irradiance and radiation pressure described above using classical electromagnetics. And again, reflected or otherwise emitted photons will contribute to the net radiation pressure identically.