Why does toothpaste reappear?
Why does toothpaste reappear after I've wiped it off?
Dave - I get in trouble with this all the time with my girlfriend because I'm pretty clumsy when doing my teeth as well. What's going on here is to do with the reason why T-shirts go fairly transparent when they get wet. Basically, the reason why you can see a white object is actually, if you look at it under a big microscope it's transparent crystals or lumps. When light hits it, it gets bent because light goes slower in it. It gets bounced off in all sorts of different directions because the surface is very rough. If you cover it with water that smooths out the surface and so light can get through much better so it looks dark. As soon as that water dries out - so you've actually cleaned up most of the toothpaste but not quite all of it, then you can see those little particles, light bounces off them and it looks white.
Chris - A bit similar to why paint looks lighter when it dries.
Dave - Exactly the same thing.