colourful volume caustics!

[mrCarnivore]

@DaveC: Now, that is not correct. Beeing not in the visible spectrum ONLY means that there is not one single wavelength of light that has that specific colour. All pure colours have one distinct wavelength except most shades of violet (purple). White for example is visible as well but also does not have a single wavelength, since it is a mixture of colours. So is purple...

In the CIE diagram the purple colours lay on the so called purple line.

http://en.wikipedia.org/wiki/Image:CIExy1931.png

The numbers on that plot are the respective wavelengths of that colour (only valid on the outer rim of the curve. All other colours are mixtures of different wavelengths. Notice the shades of purple that connect the end of the blue to the end of the red colours (purple line). They do not have distinct wavelengths.