Interference is a characteristic phenomenon exhibited by waves. When two electromagnetic waves interact with their peaks and troughs coinciding, a resulting wave with enhanced amplitude is produced. This is known as constructive interference. In this case, the two waves interacting are in phase with each other.
Alternatively, if the two waves coincide and interact in a way such that the trough of one wave coincides with the peak of the other (in an out of phase manner), the resultant wave will display a much lower amplitude. This is known as destructive interference.
Waves also show a characteristic behavior called diffraction. When a beam of light passes through a slit with a size comparable to the wavelength of the incident beam － the beam bends (or diffracts) around the slit. On the contrary, when a stream of particles is passed through a slit, the particles simply come out through the aperture.
Further, when a beam of light passes through a pair of closely spaced slits separated by a distance comparable to the wavelength of light, circular waves are produced at each slit by the process of diffraction. These two waves interfere with each other such that an interference pattern with alternate dark and bright lines is obtained on a screen placed at a short distance behind the slits.
The bright-line is produced at the center of the screen as the two waves travel an equal distance to reach this point and interfere constructively. When the two waves travel the small distance away from the center in either direction, they travel slightly different distances. They are out of phase. When the difference in their traveled distances is exactly one half of the wavelength, they meet by producing destructive interference. The dark regions correspond to regions where the peaks for the wave from one slit happen to coincide with the troughs for the wave from the other slit (destructive interference), while the brightest regions correspond to the regions where the peaks for the two waves (or their two troughs) happen to coincide (constructive interference). The diffraction pattern is an inherent property of waves and presents compelling evidence for the wave nature of light.
This text is adapted from Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy.