RAYLEIGH scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. Wikipedia
I know you're right Baldric- our posts crossed but I was just pointing out to Dave that after 80 posts without details of letter counts, and some without clues, he may want to look through the guidelines and this may help him get answers more quickly