Why do our blood vessels appear blue?

Q. Why do our blood vessels appear blue?

Ans. Our blood vessels usually appear blue in spite of the fact that the blood flowing in them is red in colour. The reason is all attributed to the penetration and scattering abilities of the different wavelengths of light. Blood appears red because the hemoglobin (oxygen carrying proteins) in red blood cells absorbs all the wavelengths of light except the red one, which gets reflected back to make the blood appear red. On the other hand, to illuminate the veins, light has to penetrate the skin. When the light of different wavelengths hits the skin, most of the higher wavelengths are absorbed by the skin whereas the shorter wavelength, i.e. the blue part of light does not penetrate the skin and gets scattered and reflected back to our eyes making the veins appear blue.