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Why does the double slit experiment work for natural light?

• The original double-slit experiment worked with natural light. However, the explanations for the interference pattern make the most sense for coherent light such as a laser. In fact, laser light is usually recommended for the experiment. Why does natural light still interfere in the same way? The explanation I am referring to uses the fact that coherent light contains photons with all the same phase, making it clear that the light waves coming through each slit can interfere with one another in a reliable way. My understanding is that natural light has photons of all different phases, so we would not expect the light coming through one slit to have a phase that matched up with the phase through the other slit in any particular way. Yet, a predictable interference pattern is still observed. The only explanation which I can think of is that each photon is passing through both slits and interfering with itself. I find this a bit difficult to believe, particularly since the experiment works in open air; but perhaps it is true.

• Answer:

  Interference phenomena will still happen with incoherent light, provided the range of wavelengths with significant amplitude is not too large, but the interference fringes will appear more indistinct and washed out.  This is because each wavelength of light will individually interfere and if the spread is not too great then the fringes will be at approximately the same locations, so when you add them all together you will still get a pattern. The original Young's slit experiment was done with sunlight.  This has an approximately black-body spectrum.  The width of the blackbody spectrum goes down with the temperature of the source, so using a high temperature source like the sun is better than any of the other light sources that were available at the time, such as candles.