I'm studying wave interference and I have a few questions regarding it that I can't quite figure out: 1.) I was once taught that for waves to interfere, they have to be of the same frequency, but then how can beat interference occur? What are the actual requirements for interference? 2.) As far as I know, for us to perceive beats, the difference between the frequency of the two waves can't be more than about 40hz, but then how is it possible to hear the beats when tuning a guitar? The two E strings on a guitar are one octave apart, right, which means one has double the frequency of the other, which is certainly more than 40hz? 3.) When two sound waves with slightly different frequencies interfere to create a beat, the "frequency" of the resulting wave is said to be the difference in frequency between the two waves that interfered; so wave A minus wave B. But that "frequency" is really variations in the loudness of a new wave with some other frequency. But what is that frequency, is it wave A's frequency plus wave B's frequency? 4.) Can the phenomenon of beat interference occur for light as well as sound? If it can, then why don't we see it? Thanks so much! This is such a great community and I thank all of you who contribute to it and help students like me.