Photon diffusion

Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random walk. A large ensemble of such photons can be said to exhibit diffusion in the material, and can be described with a diffusion equation.

Astrophysics

In astrophysics, photon diffusion occurs inside a stellar atmosphere. To describe this phenomenon, one should develop the transfer equation in moments and use the Eddington approximation to radiative transfer (i.e. the diffusion approximation). In 3D the results are two equations for the photon energy flux:

where is the opacity. By substituting the first equation into the second, one obtains the diffusion equation for the photon energy density:

Medical science

In medicine, the diffusion of photons can be used to create images of the body (mainly brain and breast) and has contributed much to the advance of certain fields of research, such as neuroscience. This technique is known as diffuse optical imaging.

gollark: Very unrelated to anything, but I recently read about how TV licensing works in the UK and it's extremely weird.
gollark: "I support an increase in good things and a reduction in bad things"
gollark: Or maybe they just check it for keywords automatically, who knows.
gollark: I assume most people would agree with (most of) those things, but just saying, effectively, "more good things, fewer bad things" isn't very meaningful. Maybe that's what you're going for, but I assume they might want you to say/make up more personal-scale things.
gollark: > a return to traditional moral valuesSounds vaguely worrying. But otherwise yes, I suppose. But that's not exactly... actionable?

See also


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