How is energy conserved during interference of light waves? I already know a textbook explanation that in constructive interference the resulting intensity is four times the initial intensity and for destructive interference it is zero,so the net energy in youngs double split experiment remain constant.But shouldn't energy be conserved in any closed system we consider? We can consider two rays that interfere constructively alone as a system.Also existence of bright fringes does not imply existence of dark fringes, I could add glass pieces over the region of board where destructive interference was supposed to be and prevent destructive interference to an extent and vice versa.Will energy be conserved in such a system?