according to book "....Let us look at a particular wave crest generated at the left end of the string by a fork. It travels to the right end of the string, is reflected back, and is reflected again at the fork.Now it interferes with the next wave...." doubt: the fork keeps vibrating and keeps producing waves. so just after it is reflected from the right end, it will interfere with the next crest right? why didthe author assume the first crest travelled 2L distance without interference? wouldn't it interfere withe next frest just after it travels a distance L?