Interference:

In the year of 1803, an English physicist Thomas Young reported the results of his famous double slit experiment on light. He observed which while a monochromatic light was made to pass by two closely spaced narrow slits and to fall on a distant screen a series of dark and light bands (known as fringes) were observed on the screen.

The origin of these fringes was explained in terms of the interference of the wavelets originating from the two slits. A bright spots were attributed to the constructive interference of the wavelets, meaning thus that the wavelets are in phase and while they interfere, their amplitudes add up to provide the bright spot. Instead, when the wavelets are not in phase they undergo destructive interference whereby these cancel every other's amplitudes and result within a dark spot. The third possibility is in which the waves are partially in phase and partially out of phase. In like a situation the resultant amplitude is the sum of the amplitudes of the two wavelets at different times or distance.

Figure: Schematic representation of interference of waves