Part of the show Science Q&A Show
Felix in Dorset asked:
Why is it that when you look at the sun or a bright light with your eyes closed, your eyelids are red? Is it because of your blood or because of colours being absorbed by red?
It's a bit of both, because there are two things going on here. The skin on your eyelids has a very rich blood supply and is also very thin, which allows some light to pass through. Blood looks red because of the iron in haemoglobin, which absorbs all colours of light except red (which it reflects). Some of that red light is reflected out of the blood in your eyelid and into your eye, which therefore accounts for some of the red you can see. The second part of the tale is that tissue transmits red wavelengths of light very well, but it doesn't transmit blue or wavelengths of light towards the blue end of the spectrum very well. So what happens is that light hits your eyelid, and blue light is absorbed by the skin, while the red light travels through. By the time the light travels through your shut eyelid and enters your eye, it's mainly red light that's getting into the eyeball, which is why you see red.