A close shave
Yesterday I was having my weekly shave when the shaving foam got me thinking. Why is it blue when it comes out of the can but white when up put it on my chin? The blue colour must be caused because the gel absorbs all the colours in the light except blue which it reflects. When the gel turns into bubbles the surface of each bubbles reflects all of the colours so the foam looks white. so the surface reflects all colours but if reflected from the body of the substance it only reflects blue. The same effect explains why crushed brown sugar looks white. So why are some powders coloured? Is it because some substances are coloured due to surface reflections and others due to reflections in the body of the material?
Maybe Geoff knows?