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# Interference

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

German: Interferenz

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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:

• spatial and temporal overlap of the two light fields
• phase coherence of the two light fields
• non-orthogonal polarization states 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.

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. If you like this article, share it with your friends and colleagues, e.g. via social media: