# What excactly is a "fourier component of a density fluctuation"?

Light scattering texts say depending on the scattering angle, you are seeing a certain fourier component of a density fluctuation. This density fluctuation varies sinusoidally due to Brownian motion of particles. I have a few questions to help my understanding:

Why does Brownian motion cause sinusoidal density fluctuations? For instance, the diagram on page 124 of this book on light scattering and colloids, shows this pictorally, but I cant understand why the Brownian particles would ever be like that?

At any moment in time isn't the position of each particle just in a random place?

How can I think of a "Fourier component of a density fluctuation?" What exactly is a "Fourier component" in this context.

Thanks

• Maybe this has a link with the Wiener-Kitchin Theorem. It is not because at a given time, the position is random, that you can't have correlation between this position and the position at a later time. For example for a stationnary process $x(t)$, we define the autocorrelation as $C (t_{1} - t_{2}) = \left< x(t_{1}) x(t_{2}) \right>$. Note that $C$ only depends on the time-shift $t_{1} - t_{2}$. What is important is the power-spectrum of the correlation, given by $C(\omega) = \int d t C (t) e^{- i\omega t}$. Hoping it helps...
– jibe
Aug 6, 2014 at 19:57