Question: Can only certain light be used for certain things?e.g. the way that you can only use red light in a dark room and the way that leaves don't absorb green light?

Susan Cartwright answered on 16 Jun 2015:

Yes, yes, yes!
The energy carried by light depends on its wavelength: longer wavelengths, like red, carry less energy, while shorter wavelengths, like blue, carry more energy. You use red in a darkroom because the low-energy red light does not affect the photographic film, so you won’t fog the pictures you are trying to develop: the chemical changes that cause the image to be produced require higher energy light. Likewise, the photomultiplier tubes that Super-Kamiokande uses to detect light only see blue and green, not red – fortunately, Cherenkov light is blue, so that’s OK.

A classic example is the photoelectric effect, where shining light on the surface of a metal causes electrons to be emitted. Every metal has a limiting wavelength of light: shorter wavelengths will cause the electrons to be emitted, longer wavelengths won’t. This effect is what made Einstein realise that light is made of particles called photons, rather than just being a continuous wave. Solar cells rely on the photoelectric effect, and a key way of improving their perforamcne is to try to tune their limiting wavelength to use as much solar energy as possible: too low, and the red light will be unused; too high, and the “extra” energy carried by the blue light will be wasted.

What an excellent question.