Blessed by the Potato

So I was holding my hand up to a really bright light, and noticed that the light went right through my hand as though the bone wasn’t there. That diffuse red glow is nothing new, but what I noticed is that the blood vessels were blocking the light. “No way,” I thought, “blood can’t be more opaque than bone!”

Of course, that’s not really the case. Instead, it has to do with scatter: inside the tissue, the light isn’t going straight through like with something that’s actually transparent. Rather, the light is being scattered: bounced off molecules many times before making it out the other side. When something is highly scattered, then the light coming out often has very little relationship to the direction it came in, and it can even scatter around obstructions, such as bone. To get that uniform red glow, you need a fair bit of tissue, enough so that the average scatter length (how far the light can penetrate in the tissue before it finds something to scatter off of) is within the tissue.

With the blood vessels, they’re close to the surface, so when they block the light there isn’t enough tissue for the light to scatter around them (like there is for the bones), which makes them look opaque!

I know that the stock market posts have gotten out of control here lately, and I think it’s wearing on my regular readers (all three of them), since the comment rate has dropped off a cliff. For the first two years of BbtP, there were on average two comments per post (granted, one of those was probably me responding to the first comment), but lately it’s averaged four posts per comment, so to help get things back in balance, I declare this an open comment thread! Comment about anything you want, post any questions or requests you have, whine about whatever’s on your mind, or pimp your own blog (not that it’s going to help you, to be honest, but go ahead).