# Does diffusion happens for the photons from higher concentration to lower concentration

I understand that it is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration and the diffusion will continue until the concentration gradient has been eliminated.

My question is, do light photons behave the same or not?

For example, if I put two light sources of the same power in a vacuum at a finite distance, will I find that it's the same probability of finding a photon when the distance measured from the sources is the same.

• Not likely. For a diffusion motion, the momentum of a particle should be randomiized (scattered) within a reasonable samll legth scale and time scale. It is not likely to be applicable to the photon case. – ytlu Mar 19 at 8:39
• Looking at a cloud outside my window. The light I see is photons that have entered the cloud and diffused back out. – John Doty Mar 19 at 17:32
• @JohnDoty Should scattering be considered the same as diffusion? – BioPhysicist Mar 19 at 17:41
• @BioPhysicist Diffusion is a consequence of scattering. – John Doty Mar 19 at 17:45

No. The light waves (and the associated photons) will just keep on going in a straight line away from each source. In the case of a source emitting equally in all directions, the light intensity falls with distance squared from a given source, so for two such sources in otherwise empty space the net intensity at some point $$\bf r$$ is $$P = \frac{P_A}{({\bf r} - {\bf r}_a)^2} + \frac{P_B}{({\bf r} - {\bf r}_b)^2}$$ if the sources are at $${\bf r}_a$$ and $${\bf r}_b$$ and $$P_A$$, $$P_B$$ is the intensity at unit distance from each source respectively.
• You should clarify that $\frac{1}{r^2}$ is for a point source, or for a source sufficiently distant that it can be treated as such – Carl Witthoft Mar 19 at 13:20