constructive interference
To understand the double slit interference pattern, we consider how two waves travel from the slits to the screen, as illustrated in Figure 27.13. Each slit is a different distance from a given point on the screen. Thus different numbers of wavelengths fit into each path. Waves start out from the slits in phase crest to crest , but they may end up out of phase crest to trough at the screen if the paths differ in length by half a wavelength, interfering destructively as shown in Figure 27.13 a . If the paths differ by a whole wavelength, then the waves arrive in phase crest to crest at the screen, interfering constructively as shown in.