well actually not to get scientific and ruin the fun but its all based on wavelengths.. and when you go to the dr to get your ice checked that is pretty much waht he is checking is how well your eyes can interprit those wavelengths.. a human eye can only see a small scall of the spectrum called the "visible spectrum" above it you have ultra violet and below you have things like radio waves.. it also has to do with pigment which change the regular suns light into a different wavelength, as it is reflected off this small particle, into a diff wavelenth..
well that is what my damn biology teacher told me anyways.. but hell the guy is weird