Wave interference

In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference.

For example, when two pebbles are dropped in a pond, a pattern is observable; but eventually waves continue, and only when they reach the shore is the energy absorbed away from the medium.

If the difference between the phases is intermediate between these two extremes, then the magnitude of the displacement of the summed waves lies between the minimum and maximum values.

Consider, for example, what happens when two identical stones are dropped into a still pool of water at different locations.

Interference of light is a unique phenomenon in that we can never observe superposition of the EM field directly as we can, for example, in water.

Superposition in the EM field is an assumed phenomenon and necessary to explain how two light beams pass through each other and continue on their respective paths.

Prime examples of light interference are the famous double-slit experiment, laser speckle, anti-reflective coatings and interferometers.

This represents a wave at the original frequency, traveling to the right like its components, whose amplitude is proportional to the cosine of

A simple form of interference pattern is obtained if two plane waves of the same frequency intersect at an angle.

If the light from two point sources overlaps, the interference pattern maps out the way in which the phase difference between the two waves varies in space.

When the plane of observation is far enough away, the fringe pattern will be a series of almost straight lines, since the waves will then be almost planar.

Interference occurs when several waves are added together provided that the phase differences between them remain constant over the observation time.

Some of the differences between real valued and complex valued wave interference include: Because the frequency of light waves (~1014 Hz) is too high for currently available detectors to detect the variation of the electric field of the light, it is possible to observe only the intensity of an optical interference pattern.

Quantum mechanically the theories of Paul Dirac and Richard Feynman offer a more modern approach.

The discussion above assumes that the waves which interfere with one another are monochromatic, i.e. have a single frequency—this requires that they are infinite in time.

Two identical waves of finite duration whose frequency is fixed over that period will give rise to an interference pattern while they overlap.

However, single-element light sources, such as sodium- or mercury-vapor lamps have emission lines with quite narrow frequency spectra.

[5] All interferometry prior to the invention of the laser was done using such sources and had a wide range of successful applications.

In wavefront-division systems, the wave is divided in space—examples are Young's double slit interferometer and Lloyd's mirror.

Quantum interference concerns the issue of this probability when the wavefunction is expressed as a sum or linear superposition of two terms

In this experiment, matter waves from electrons, atoms or molecules approach a barrier with two slits in it.

The interference pattern occurs on the far side, observed by detectors suitable to the particles originating the matter wave.

The experiment played a major role in the general acceptance of the wave theory of light.

Richard Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.

[12] The results of the Michelson–Morley experiment are generally considered to be the first strong evidence against the theory of a luminiferous aether and in favor of special relativity.

Sixty years later, in 1960, the metre in the new SI system was defined to be equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

This definition was replaced in 1983 by defining the metre as the distance travelled by light in vacuum during a specific time interval.

All of the telescopes in the array are widely separated and are usually connected together using coaxial cable, waveguide, optical fiber, or other type of transmission line.

An acoustic interferometer is an instrument for measuring the physical characteristics of sound waves in a gas or liquid, such velocity, wavelength, absorption, or impedance.

The waves strike a reflector placed parallel to the crystal, reflected back to the source and measured.