With regards to the photoelectric effect the wave theory does not match up to observations of instantaneous emission of photoelectrons. Now my justification for this (not stated anywhere in my textbook or by my teacher) is that : Intensity is a measure of energy moving past unit area per unit time, correct ? Since a wave's intensity is proportional to amplitude2 there are periodic lows and highs in energy that a specific electron is receiving. Is this correct ? My one problem with this justification of mine is that I'm making use of amplitude when it is the displacement varying form one moment to the next. :/ Could someone supply me with a good explanation ? Please, remember that I am an A Level/High School student.