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Interference

Definition: a range of phenomena associated with the superposition of waves

German: Interferenz

Categories: general optics, physical foundations

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Interference is an optical effect which can occur when two or more light beams are superimposed. More precisely, for interference to occur, several conditions have to be met:

interference
Figure 1: Standing wave interference pattern (showing the optical intensity) from the superposition of two elliptical Gaussian beams under some angle.

In this case, the resulting total light field does not have an optical intensity which equals the sum of the intensities of the superimposed beams. Instead, its complex amplitude is the sum of the amplitudes of the superimposed beams. For example, the amplitudes of two equally intense light beams can have opposite signs at some location, so that they can cancel each other (destructive interference). On the other hand, with equal signs (equal phases) of both contributions (constructive interference), the total intensity can be four times that of the single beams. Nevertheless, the total energy is conserved in any case. For example, if two light beams of equal intensity, frequency and polarization are superimposed on a screen with some angle between the beams, an interference pattern occurs which consists of bright and dark stripes (see Figure 1). It is called a standing wave pattern, since the minima and maxima of the total optical intensity can stay at their positions, although the optical waves are moving with high velocity.

interference of circular waves
Figure 2: Snapshot of the superposition of two circular waves.

Figure 2 illustrates the superposition of two circular waves with the same frequency but different source points. It shows a snapshot, i.e. the field distribution at one particular moment in time. As time progresses, the spatial patterns move away from the point sources.

By averaging the optical intensity corresponding to this pattern over one oscillating period, the interference pattern in Figure 3 is obtained.

interference of circular waves
Figure 3: Time-averaged intensity pattern.

Interference effects also occur in multimode fibers. Figure 4 shows the simulated output intensity profiles of a multimode fiber when an input beam is scanned across its input face.

intensity profiles at the end of a multimode fiber
Figure 4: Intensity profiles at the end of a multimode fiber, shown as animated graphics. A Gaussian input beam is scanned through the horizontal line (slightly above the center of the fiber core). This model has been made with the RP Fiber Power software and is described in more detail on a separate page.

Importance of Interference Effects

The phenomenon of interference is of great importance in optics in general, and also in laser physics. Some examples:

Questions and Comments from Users

2020-08-31

What the difference between coherence and interference?

Does interference occur between two waves of different frequency?

Answer from the author:

Coherence is a property of light, interference a phenomenon. Coherence is often a prerequisite for observing interference.

If two light waves have quite similar frequencies, one may observe interference. Frequently, the optical frequency differences are so high that the correspondingly high beat frequencies prohibit the observation of interference.

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See also: interferometers, optics, optical phase, coherence, holography, beat note, The Photonics Spotlight 2007-09-27, The Photonics Spotlight 2009-05-22
and other articles in the categories general optics, physical foundations

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