# Dynamic Light Scattering

In the Dynamic Light Scattering experiment, how is the intensity distribution plotted against time, keeping in view the multiple scattering of photon particles in laser ?

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Can you clarify your question? Industrial particle size measuring kit requires you to use low particle densities so multiple scattering is minimal. Are you asking how you can analyse the scattering for higher densities where multiple scattering is significant? – John Rennie Jun 2 '12 at 8:15
yes please. What is the difference between both the processes,i.e. in single scattering and multiple scattering ? Also please clarify that in single scattering if a photon scattered by a particle in the solution gets scattered again due to another particle in its way how is this going to affect the distribution ? – stp30 Jun 4 '12 at 4:25

## 1 Answer

Single scattering is when the light comes in, encounters a particle, scatters off it, and leaves. Multiple scattering is when the light comes in, scatters off one particle, then scatters off a second particle before leaving. Multiple scattering isn't good. If you draw rays from your light source and your detector, then your scattering volume is the volume where those two rays intersect each other. Those two rays define a plane. If you only have single scattering, then you know all of the scattering that you see at the detector came from that scattering volume in that plane. If you have a significant amount of multiple scattering, you also see some scattering events where a photon hits a particle in the scattering plane, scattered out of the plane, then scattered back into your plane. You no longer have a well-defined scattering volume.

There is another, less problematic issue that shows up even before your concentration gets high enough to worry about multiple scattering. Usually, the particles are approximated as an ideal gas in that they don't interact with each other. At low enough concentrations, that is a good approximate, but it begins to fail at higher concentrations. This effect is useful in static light scattering, where it allow you to measure the virial coefficients of the particles. I don't think it's useful in dynamic light scattering. I don't know if it causes problems for dynamic light scattering. It might.

Typically what's plotted in dynamic light scattering is the magnitude of the autocorrelation of the scattering on the y-axis, and the decay time $\tau$ on the x-axis. $\tau$ is a parameter used in the definition of the autocorrelation function. The autocorrelation function is a precise definition of how correlated the scattering signal at time $t$ is with the scattering signal at time $t+\tau$. The autocorrelation varies with $\tau$, so it is plotted against $\tau$

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