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?
When hair gets wet it gets closer together. Hair is light when it's dry for the same reason that glass is clear when it's as a glass but lighter when it's made into tiny particles of sand. What's happening when it's dry is that light is bouncing off all the surfaces of the hair and bouncing around all over the place and reflects lots of wavelengths of light and looks lighter. When the hairs are covered with water it smooths the surfaces, more light can go through deep into the hair rather than be reflected back, so it looks darker.