- A simple form of interference takes place when two plane waves of the same frequency intersect at an angle.
- Light interference is often evident in the appearance of interference patterns such as those that produce supernumerary rainbows.
In general terms, interference patterns are produced by a process of energy redistribution. In the case of waves on a pond, the energy gained through constructive interference is lost as a result of destructive interference.
- Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The outcome is that the amplitude of the resulting wave is the sum of the amplitudes of the original waves.
- Destructive interference occurs when the trough of one wave meets the trough of another wave. The outcome is that the amplitude of the resulting wave is equal to the difference in the amplitudes of the original waves.
Interference of light is a unique phenomenon in that we can never observe superposition of the EM field directly. Superposition in the EM field is an assumed and necessary requirement, fundamentally 2 light beam pass through each other and continue on their respective paths. Light can be explained classically by the superposition of waves, however a deeper understanding of light interference requires knowledge of wave-particle duality of light which is due to quantum mechanics. Prime examples of light interference are the famous double-slit experiment, laser speckle, anti-reflective coatings and interferometers. Traditionally the classical wave model is taught as a basis for understanding optical interference, based on the Huygens–Fresnel principle however an explanation based on the Feynman path integral exists which takes into account quantum mechanical considerations.