Why do we see our veins blue?
The reason why we see our veins blue is complex. First, we need to remember all the ways light (that is, electromagnetic radiation) can interact with matter: it can be reflected / scattered, absorbed and transmitted. The light-matter interaction actually depends on the wavelength of the light. Some objects will therefore absorb certain wavelengths and reflect others. This is why we see different objects having different colours: what we see is the light that is reflected or scattered from them into our eyes.
Indeed, blood appears red because it absorbs the part of the white light spectrum that is not red (from orange to violet), and so it reflects and scatters only red light. It is the case for both oxygenated and deoxygenated blood, as shows the absorption spectrum below.
So, blood appears red and veins appear red as well when they are very close to the skin surface, but blue when they are at a deeper depth. Why? To understand this phenomenon, let’s have a further look at the absorption spectrum above. It also shows light absorption by melanin (those pigments in our skin that protect us from sun light because they absorb it, especially the high-energy UV radiation, and then dissipates the absorbed energy as heat in a harmless process for our cells), and light scattering by the skin. We see that both will absorb and scatter light slightly more in the “blue” region (in the following, “blue” will mean “not red”). It means that blue light penetrates less deeply into our skin than red light.
A little and easy experiment can be carried out to illustrate this: in a dark room, put your hand on a lamp and look at the light transmitted through your hand: it is red (see picture).
Now, we need to take into account another element: blood does absorb red light as well (look back at the spectrum), even if indeed much less than blue light. But in our story of the veins that appear blue, it is very important. And we can see that on the picture of my hand (by the way, it’s much clearer in reality than on the picture): the black arrow indicates a vein. It appears darker because it has absorbed some of the red light that was transmitted through my hand (again: the lamp emits white light but the blue part has been either absorbed or scattered by the skin and so only red is transmitted through).
So, we know now that blue light penetrates less deeply than red light into our skin, which means that blood veins receive mainly red light. And as blood slightly absorbs red light, it means that with respect to the zones without veins, the veins will scatter a light back to our eyes that is slightly “less red”. And the last phenomenon that explains why we see this “less red” light as blue is an optical illusion — a sort of trick done by our brain! And it is simply that our brain doesn’t really perceive colours “objectively” but instead its perception depends on the other colours around. Look for instance at this London tube map: do you see the same yellow colour for the circle line at the top and at the bottom (see the black arrows)?
So, our brain does the same with our slight less red veins with respect to the environment, and we see our veins blue.
