Due to the different optical path length between a-b-d path and a-c-d path in Fig. 1, the overlapped light waves interfere with each other at point d. If the phase shift is expressed as (2n+1)π, the two light waves are totally out of phase, so that destructive overlap occurs. On the other hand, if the phase shift is expressed as 2nπ, two waves are constructively overlapped to give bright area on the screen. The wavefunction which describes the quantum state of particles is also a kind of wave, so that we can observe the constructive and destructive overlaps. Such phenomena is called "quantum interference" compared to the normal interference of classical waves.