In Young's double slit experiment a single source is used to illuminate two slits which then acts as two coherent sources to produce interference pattern. But, what if I put color filters on the two slits. Will it make the slits incoherent?

I myself think yes because in one of my previous questions here I got to know that two different sources cannot be coherent. Put filters will make the slits act like two different sources which are not coherent as light is coming through two different filters.

Is my thinking correct?

  • $\begingroup$ Colour filters also makes the two superposing waves have different frequencies/ wavelengths, which does not allow a temporally and spatially stable interference pattern to develop. $\endgroup$ Mar 4, 2014 at 10:21
  • $\begingroup$ @SatwikPasani What if I put two blue color filters on the two slits? Then the light emitted would have the same frequency. $\endgroup$ Mar 4, 2014 at 10:24
  • $\begingroup$ But if you put identical filters on a common source, then you still have a single source. What it really depends on is using filters which do not disturb the wavefront--any glass which is not "hazy" will satisfy this condition. $\endgroup$ Mar 4, 2014 at 12:42
  • $\begingroup$ @CarlWitthoft Suppose I have used two blue filters it will absorb all colors and transmit only blue light through it. But, will this be identical to using a source(i mean the single source which is used to illuminate two slits.) which produces monochromatic blue light? $\endgroup$ Mar 4, 2014 at 13:09
  • $\begingroup$ If your source is coherent (unlikely if it's not monochromatic to begin with), then the filtered light will remain coherent. $\endgroup$ Mar 4, 2014 at 14:02

1 Answer 1


This will not happen because color filters don't work like this.

A red color filter does not convert blue photons to red photons. It absorbs photons that are not red (most of them) and lets red photons pass unaffected. If you use a red filter for one slit then a blue photon will not go through that slit at all, so you fill effectively have a single-slit experiment.

You may choose the filters such that a photon has a certain probability to pass through both slits. Such photons will interfere same as they would without a filter.

  • 1
    $\begingroup$ It will have a small effect on the photons in practice, due to having a different refractive index than air. (But that's a minor technical issue that's easily mitigated, e.g. by putting a transparent piece of the same material over the other slit.) $\endgroup$
    – N. Virgo
    Mar 13, 2014 at 9:28

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