Coherent Sources And Sustained Interference

Interference happens all the time, every place at every moment. Yet we don’t see interference patterns everywhere. This is because light waves are randomly generated every which way by most sources. This means that light waves coming out of a source do not have a constant amplitude, frequency or phase. For, example incandescent bulbs generate a wide range of frequencies of light including all colors of the rainbow. Moreover, the light coming out of the bulb is randomly generated every moment in all directions. This means that the starting point of the wave generated may be a maximum, a minimum or any point in between. There is no way of predicting which phase the wave will start. Such a source is said to be incoherent.

What are coherent sources ?


Two sources are said to be coherent when the waves emitted from them have the same frequency and constant phase difference.

Interference from such waves happen all the time, the randomly phased light waves constantly produce bright and dark fringes at every point. But, we cannot see them because since they occur randomly. A point that has a dark fringe at one moment may have a bright fringe at the next moment. This cancels out the effect of interference effect and we see only an average brightness value. The interference is not said to be sustained since we cannot observe it.

However, to truly observe interference, we need coherent sources of light. Coherent sources have the following characteristics:

  1. The waves generated have a constant phase difference
  2. The waves are of a single frequency

Young’s Double Slit Experiment

Young's double slit experiment

In the Young’s double slit experiment, two coherent sources were generated using diffracted light from a single slit. Note that, the waves must have a constant phase difference, so the two slits need not be placed symmetrically from the first slit to observe an interference pattern.

Lasers are commonly used as coherent sources and use a phenomenon called Simulated Emission to generate highly coherent light. Small sources of light are at least partially coherent. This is why we can observe interference patterns on soap bubbles and appreciate the iridescence of butterfly wings. While sunlight is incoherent overall, small portions on small areas are generally partially coherent.

For sustained interference to occur, the following conditions must be met:

  1. Coherent sources of light are needed.
  2. Amplitudes and intensities must be nearly equal to produce sufficient contrast between maxima and minima.
  3. The source must be small enough that it can be considered a point source of light.
  4. The interfering sources must be near enough to produce wide fringes.
  5. The source and screen must be far enough to produce wide fringes.
  6. The sources must emit light in the same state of polarization.
  7. The sources must be monochromatic.

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Practise This Question

When one of the slits in Young's experiment is covered with a transparent sheet of thickness 3.6×103 cm the central fringe shifts to a position originally occupied by the 30th brigth fringe. If λ=6000A, the refractive index of the sheet is