So in Young's Double slit experiment the source of light that passes through the double slit must be monochromatic and a coherent source (from my undertanding, this is to get a uniform pattern projected on a screen/wall/whatever, correct me if I am wrong). Young without having access to lasers used a light filter to filter light for it be of one wavelength and then exposed it to a single slit for a coherent source. From what I understand, a coherent source of light is a wave which has a constant phase/path difference, meaning that at any point on the wave there is the same phase/path difference as any other point on the same wave. However, I can't figure out why diffracting a monochromatic source of unpolarised light would cause the resultant wave to be coherent, as I am unable to see where phase/path difference are relevant.

Any help is appreciated. Thanks!

  • $\begingroup$ Hmm ... people who do coherent optics seriously make a strong distinction between temporal coherence and spacial coherence. The pinhole will generate the latter but the not the former. $\endgroup$ Aug 26, 2019 at 22:53
  • $\begingroup$ There is no need for monochromicity to observe diffractive effects, but the pattern is much simpler and easier to explain if you have this property. $\endgroup$ Aug 26, 2019 at 22:54
  • $\begingroup$ "must be monochromatic and a coherent source". This is not required. You will see superimposed shifted and wavelength dependent patterns if the incoming light is not collimated and monochromatic. $\endgroup$
    – my2cts
    Aug 26, 2019 at 22:59

1 Answer 1


The single slit is in order to generate a point source in two dimensions.

A point source, by construction will allow only specific wavelengths to its size to go through and spread radially. This means that the wavefront can be described mathematically with sinusoidal functions with fixed phases. For details look at this link.


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