Young's Double Slit Interference

In the double slit experiment, a single source is split in two,
to generate two coherent sources.

When the light from the two sources is projected on a screen,
an interference pattern is observed.

To explain the
origin of the interference pattern, consider the distance
traveled from the two sources. At the center of the
screen the waves from the two sources are in phase. As we
move away from the center, the path traveled by the light
from one source is larger than that traveled by the light
from the other source. When the difference in path is
equal to half a wavelength, destructive interference
occurs. Instead, when the difference in pathlength is
equal to a wavelength, constructive interference occurs.