Why are coherent sources necessary to produce a sustained interference pattern?
Interference of light
The phenomenon of interference is nothing more than the phenomenon of the superposition of two or more waves, resulting in the increase or decrease of the amplitude of the resulting wave. In extreme cases the amplitude of the resulting wave is the sum of the amplitudes of the superposed waves (constructive interference), or is zero (destructive interference).
Answer and Explanation:
When it comes to light it is exactly the same, only that the light sources must be coherent with each other. It is said that two light waves are coherent when they have the same wavelength (temporal coherence) and the separation between the sources that produce them is constant over time (spatial coherence). If these requirements are not met, the interference phenomenon cannot take place.
A classic interference pattern is that corresponding to Young's experiment (double slit experiment). In this experiment, a light wave front is divided into two different fronts by passing the light through two slits separated a tiny distance from each other. This produces two new light sources that have the same wavelength (temporal coherence is guaranteed). In addition, as the slits are fixed, their distance will remain constant over time (spatial coherence is guaranteed). These requirements ensure that the phase difference between the two new wave fronts is constant, so it is possible to produce an interference pattern.
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from CLEP Natural Sciences: Study Guide & Test PrepChapter 8 / Lesson 16