This phenomenon is actually used to help medical personnel find veins to take blood—by shining red, and sometimes infrared (which is an even longer wavelength) light on the arm.

Blue light has a short wavelength (about 475 nanometers), and is scattered or deflected much more easily than red light. Because it’s easily scattered it doesn’t penetrate so far into the skin (only a fraction of a millimeter). When blue light hits the skin, it’s mostly deflected back.

If you shine a blue light on your skin, what you see is basically blue skin, and veins are hard to find. You may have seen blue light used in spaces such as public bathrooms to discourage intravenous drug use.

So, now imagine the red light and the blue light shining on your skin at once, as happens when you are under white light. You will have a mixture of red, blue and other colors reflected back where there are no veins. Where there are veins, you will see relatively less red, and relatively more blue compared to the surrounding skin.

This means your veins will appear blue compared to the rest of your skin.

Interestingly, the effect varies depending on how deep the vein is, and also on how thick the vein is. Very narrow veins close to the surface, such as the capillary bed, will not appear blue.

Blue veins appear more prominent in very pale skinned people, and this may have given rise to the expression “blue blood” for European nobility in the 19th century. These people were untanned from manual labour, and so their veins appeared blue under the skin.

With thanks to Alison Gould, Science Writer at the Australian Red Cross Blood Service.

This article was originally published on The Conversation. Read the original article.

📬 Sign up for the Daily Brief

Our free, fast, and fun briefing on the global economy, delivered every weekday morning.