Light is scattered by small particles and the effect is dependent on the wavelength of the light used. If the size of the particles is much smaller than the wavelength of the light then Rayleigh scattering applies, otherwise, Mie theory is used. Light absorption can also be used to measure the concentration of a solute in a solution. The formula used is known as the Beers-Lambert Law. Data collected by these means can be used to determine the concentration of the oil particles dispersed in the water column of the Gulf in the area of the oil spill. The availability of more than one method allows cross checks to be made on the results. The method used in the reports mentioned previously to measure the presence of oil in the water column appears to be spectrofluorometry. The results of this method can be complicated by Raman scattering in which changes in the frequencies of the light emitted occur and Rayleigh scattering. Rayleigh scattering is just elastic scattering of a photon of light off a molecule and doesn't involve fluorescence in which the molecule absorbs the energy of the photon and then re-emits it. If one is not careful one can get false positives for the presence of a particular molecule. One needs to compare results with an uncontaminated sample or observe changes over time.
An interesting effect is that of critical opalescence which can be seen in this YouTube video.