- A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
- Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
- Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
- Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
- Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.

- In the field of optics, a classical wave model based on the Huygens–Fresnel principle is commonly used to explain optical interference.
- In the field of quantum mechanics, another model called the Path Integral formulation, developed by Richard Feynman, is used to explain light interference.

###### About Interference patterns

- Two waves meeting on a pond is a classic example of wave interference. When two waves overlap, their crests and troughs can interact in two ways:
- Constructive interference: If the crests of both waves line up, they reinforce each other, creating a higher crest in the resulting wave. This leads to a more noticeable wave disturbance at that point.
- Destructive interference: If one wave’s crest overlaps another’s trough, they partially cancel each other out. The resulting wave will have a smaller amplitude (lower crest) or might appear flat at that specific spot.

A two-point source interference pattern is a pattern that results from the superposition of two waves emanating from two different sources that are in phase with each other. The pattern has the following features:

- Interference fringes: The interference pattern consists of bright and dark fringes, known as interference fringes. The bright fringes represent constructive interference, where the peaks of the two waves coincide, while the dark fringes represent destructive interference, where the peak of one wave coincides with the trough of the other.
- Equidistant fringes: The interference fringes are equidistant from each other, with the distance between adjacent bright fringes (or adjacent dark fringes) being the same.
- Central maximum: The central region of the pattern is the brightest, and corresponds to the point where the two waves are in phase and reinforce each other most strongly.
- Symmetry: The pattern is symmetric about the midpoint between the two sources.
- Wavelength dependence: The spacing between the fringes depends on the wavelength of the light, with shorter wavelengths producing fringes that are closer together than longer wavelengths.
- Amplitude dependence: The spacing between the fringes also depends on the amplitude of the waves, with waves of higher amplitude producing fringes that are further apart than waves of lower amplitude.
- Angle dependence: The angle between the two sources and the observer’s position relative to the sources can affect the interference pattern, causing it to shift or distort.

###### References

- https://www.britannica.com/science/interference-physics
- https://en.wikipedia.org/wiki/Diffraction
- https://en.wikipedia.org/wiki/Double-slit_experiment
- https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle
- https://en.wikipedia.org/wiki/Probability_distribution
- https://en.wikipedia.org/wiki/Wave_function
- https://en.wikipedia.org/wiki/Wave_interference