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Photon gas
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Radiation behaves as a gas of photons.
Thermal radiation in a cavity behaves as a gas of photons with energy density u ∝ T⁴ and pressure P = u/3. Its entropy and heat capacity follow from Bose statistics.
Radiation pressure inside stars.
Photon number is not conserved.
In more detail
A photon gas, such as thermal radiation in a cavity, obeys Bose-Einstein statistics with zero chemical potential. Its energy density scales with temperature to the fourth power.
Real-world uses
- The cosmic microwave background is a photon gas at about 2.7 K.
- Stellar interiors contain significant radiation pressure from photon gas.
- A hot oven or a star's light can be described as a gas of photons, which explains its radiation spectrum.
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Frequently asked questions
Does a photon gas have pressure?
Yes, radiation pressure, equal to one third of the energy density.
Why is the number of photons not fixed?
Photons are created and absorbed by matter, so their number adjusts to temperature.
Do photons have a fixed number?
No. Their number changes as they are emitted and absorbed, so the photon gas has no fixed particle number.
Related topics in Kinetic Theory & Statistical Physics
Fermi energy · Specific heat of solids · Debye model · Planck distribution · Random walks · Equation of state of ideal gas in statistical terms