This is an interference question:
Two coherent sources of equal amplitude are emitting waves in phase. At an
equal distance away they superimposed at a point P, and hence it's
constructive interference at P. By the Principle of Superposition, the
amplitude at P is 2a.
Since intensity is proportional to square of amplitude, the intensity at P
is proportional to 4a^2. But the intensity of each wave at P is only a^2, so
the "total" intensity is 2a^2. Intensity is the energy per unit area per
unit time, so by conservation of energy, the intensity of the superimposed
wave should be proportional to 2a^2, and from superposition principle, the
intensity worked out to be 4a^2. So where has the other 2a^2 comes from? Pls
enlighten.