Part of the show Science Q&A Show
Brian in Nagoya asked:
Why is it that, even though I have light brown hair, when I get into the shower or hot spring my hair turns darker?
Hair is light when it's dry for the same reason that ice is clear when it's a sheet on a pond, but white and opaque when it's in the form of snow particles.
In dry hair, and in small particles of snow, light bounces - or reflects - off all the surfaces, returning a range of wavelengths - or colours - of light to your eye. This makes the reflected light look a lighter and brighter hue.
But when hair is made wet, this smooths the surfaces and brings the hair strands closer together. The result is that more light is guided deeper into and around the hair, rather than being reflected back towards you. Less reflected light makes the hair look darker. In the case of a sheet of ice, less light being reflected back results in greater transparency and hence it looks less white.
ls it also because water on the hair surface decreases its index of refraction and so the reflection coefficient? A lot of things become darker when wet.
I should have imagined that you were an expert of Angstrom works
No expert, just thought you would like it :)