Veins

[Alfee]

Why do veins appear either blue or purple?

[WaldoMcFroog]

Not true, I'm afraid. It's darker than freshly oxygenated blood, but not blue.

"Blood vessels near the surface are slightly translucent, as is the skin. Light from outside penetrates the skin and is reflected back out, but shorter wavelengths (ie colours from the red end of the spectrum) are more likely to be blocked by the skin on the journey out again. As a result, what you see appears bluer than it otherwise would.

"Optical physicists determined that the blueness of a vein depends on the depth of the blood vessel, its width and the blood content in the tissue around it. Others have speculated that the blue effect is also the product of a contrast with yellowy-pink skin, an optical illusion called the retinex effect."

http://whateveryourethinkingyourewrong.blogspo t.com/2008_05_01_archive.html