SCIENCE FILE

Q: Why do veins appear blue, even though the blood they contain is dark red?

A: New studies by physicist Lothar Lilge and his colleagues at the Ontario Laser and Light Wave Research Center in Toronto show that it is actually the skin that governs vein color. A vein looks blue when red light travels far enough into the skin to to be absorbed by the blood in the vein. If the blood vessel is far enough below the skin, blue light--which would normally also be absorbed by the vein--reflects out of the skin before reaching the vein, giving it a bluish cast. Lilge found that a vein must be at least 0.02 of an inch below the surface to appear blue. Veins more than 0.08 of an inch below the skin are not visible from the surface.