I am studying waves these days and my teacher just introduced Young double-slit experiment, which has always been brilliant (see picture below).

However, I have a question: why is there a single slit in the experiment?

My teacher says that the single slit is used to make sure the light at both of the double-slit are coherent (i.e. they have same frequency).

But what I think is that, since the light source is monochromatic (i.e. the light has only one frequency), theoretically, any two light rays from this source must be coherent. So it's very unnecessary to use the single slit. I believe there must be some other reason for this single slit.

Can anyone help me out? The image of Young double-slit experiment from my textbook

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    $\begingroup$ Having the same frequency of two sources does not guarantee temporal coherence, you need constant (i.e., time independent) phase difference as well between two sources. You also need spatial coherence and having a spatially small source, the pin hole, illuminating the slits is an approximation of such spatially coherent source. $\endgroup$
    – hyportnex
    Oct 17, 2021 at 16:31
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    $\begingroup$ Duplicate/Related? Why must the single slit in a double slit experiment be narrow? and links therein. $\endgroup$
    – Farcher
    Oct 17, 2021 at 22:01
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    $\begingroup$ physics.stackexchange.com/questions/675002/… See the answer to this question for more insight. $\endgroup$ Nov 5, 2021 at 12:27

1 Answer 1


The single slit is used to create light from a single source.

Even what we regard as a single source of light e.g. a bulb, can have different parts to it, e.g. different parts of the filament, emitting light at different frequencies and amplitude and varying with time in different ways.

The single source is then split into two with the two slits. That creates two sources of the same frequency and constant phase difference - i.e. coherent.

If the light from the original source landed directly onto the two slits, the two slits are not guaranteed to be coherent sources.

  • $\begingroup$ However, on the picture below, there is a filter in front of the light source, which theoretically can only let one frequency of light pass through. So theoretically, the light after the filter is already coherent. I don't understand why there is a single slit.. $\endgroup$
    – bruce mao
    Oct 18, 2021 at 13:00
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    $\begingroup$ @ bruce mao, hi. coherent also requires a constant phase difference between the two sources. If light from different parts of the filament were arriving at the two slits, the phase difference might change as in reality emission of light can stop and start, in different ways for different parts of the filament - but if it did vary but then passed through a single slit that later becomes two sources, both would vary in the same way together and so be coherent. $\endgroup$ Oct 18, 2021 at 13:52
  • $\begingroup$ Brilliant explanation. Thank you very much! $\endgroup$
    – bruce mao
    Oct 19, 2021 at 14:13

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