Question: How does light travel through the skin?

Tom Lister answered on 28 Jun 2012:

All matter is comprised mostly of space. In skin for example, if you were to take the nucleus of one atom and scale it up to the size of a football, the next nearest nucleus might be 10 miles away. So there is plenty of room for the light to get through these gaps. Inevitably, however, most of the light will have either interacted with a nucleus or with one of the electrons flying around it (which is still a couple of miles or so away from our football sized nucleus). When it does this, it may be absorbed or it may be bounce off in another direction.

About half of the light going into my skin is absorbed at some point along its pin-ball like path, the rest manages to find its way back out again.

Interestingly, blue light goes less far in skin than red, so if I shine a white torch (which produces a mixture of red, green, blue and probably most of the other colours in the rainbow) onto the end of my finger, only the red will make it through the other side.