wave 問題 10分

The meaning of coherent is actually very deep:
http://en.wikipedia.org/wiki/Coherence_%28physics%29
(but I suggest that you ignore this)

In order to demonstrate the effect of interference (I guess this is what you're investigating at the moment?) you'll need two sources of similar frequency, hence similar wavelength. It is not essential to have equal wavelengths.

from wikipedia: "Two non-monochromatic waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths."

However, in most experiments demonstrating the interference effect a single light source of monochromatic light, e.g. laser, would be used.


For your next question, I think the answer is no but I'm not 100% sure. I think sound waves would not interfere with light waves though, since if that were possible then something rather strange might happen as you shout at your computer screen...