I am wanting a full description of an apparatus used for Young's double slit (not necessarily the one that Young himself used though), which takes monochromatic light (though not laser light) and sends it through a series of slits and lenses to make it coherent and collimated (and maximising the end intensity) before finally reaching the double slit. Does anyone know any resources that will explain such a set up in detail?

Below I have included a diagram of the sort of set up I mean:

enter image description here

Although here I can explain what each component does some I am a bit unsure about and want a reliable recourse to confirm.

  • $\begingroup$ What are you basing your diagram on? There are usually no lenses between the slits or the screen. $\endgroup$
    – user263399
    Aug 14, 2015 at 13:48
  • $\begingroup$ @user263399 The first lens is to make an image of the source at the centre of the single slit. This increases the amount of light passing through the system and therefore the intensity of the final beam. Lens 2 is to collimate the pattern. $\endgroup$ Aug 14, 2015 at 13:50
  • $\begingroup$ By collimating at lens 2 your image is going to fall in the centre between the two slits is it not? $\endgroup$
    – user263399
    Aug 14, 2015 at 13:55
  • $\begingroup$ @user263399 If the centre of the single slit is at the focal point of lens 2 then as the light passes through lens 2 it will become plane, (with an image essentially been formed at infinity). This by collimating the beam the two double slits will be illuminated by a plane wave. $\endgroup$ Aug 14, 2015 at 14:00
  • $\begingroup$ These two sources give similar diagrams to the one I have given pondpol.com/appnotes/analytical/… and vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/… $\endgroup$ Aug 15, 2015 at 6:09

2 Answers 2


The company TeachSpin builds a two-slit interference instrument for use in the advanced/modern physics lab. There are several descriptions and schematics at that website.

They do not have a lens before or after the single slit. The single slit is physically close to the source.

I've used the instrument several times and it gives beautiful results for both the fine and coarse interference intensities. It also allows you to selectively block either of the two slits so that you can show that the two-slit pattern is not the sum of two one slit patterns.

  • 1
    $\begingroup$ +1 Very nice. As I said, the only purpose for the first lens is to improve the throughput. The double slits are far enough from the pinhole that the wavefront curvature has minimal effect over the lateral distance between the two slits. A pinhole close to the source works. If you have a laser, you don't really care about throughput, nor do you if you want to count photons with a PMT (you don't want too much light getting through, or you'll destroy the PMT). $\endgroup$ Aug 15, 2015 at 0:38
  • 1
    $\begingroup$ In the setup of the TeachSpin device, there is a photodiode to use wtih the laser. There is a low-intensity green-filtered current-adjustable bulb which you count with the PMT. It's a well-thought out instrument. $\endgroup$
    – Bill N
    Aug 15, 2015 at 0:45

Are you sure that the single slit between lenses 1 and 2 is a slit and not a pinhole?

If I were setting this up without a laser, I would use a pinhole below the diffraction limited spotsize of the first lens at the focus: this gives you an aberration free spherical wave at the output of the pinhole (same idea as a point diffraction interferometer / wavefront sensor), which is then collimated by lens 2 and input to the two slits. Lens 3 simply matches the diffraction pattern to the screen available, particularly if you're trying to use a CCD chip.

So in summary:

  1. Lens 1 boosts the power throughput by getting maximum power to the pinhole;
  2. Pinhole destroys wavefront phase information (by accepting only input in a smaller than diffraction limited region) thus outputting aberration free spherical wave;
  3. Lens 2 collimates
  4. Double slit: well that's the whole point of the experiment!
  5. Lens 3 matches diffraction pattern with imaging screen / CCD camera.
  • $\begingroup$ Thanks for answering, yes I do mean a pinhole then. Further more do you know any sources, or anywhere that gives this sort of information in more detail? I tried searching for 'diffraction interferometer' and 'wavefront sensors' which do seem to give better results than searching for 'double slit' although none in the detail I want (or really any that go into any detail about how the light is prepared) $\endgroup$ Aug 14, 2015 at 14:48
  • $\begingroup$ I am pretty sure that a slit will operate just like a pinhole but let more light through - I thought it doesn't matter whether you restrict the source intensity in the direction parallel to the slits? $\endgroup$
    – Floris
    Aug 14, 2015 at 19:22
  • $\begingroup$ @Floris Actually with most sources it's probably OK. The only problem arises if the source extends significantly along the direction of the slit. There can then be a significant phase modulation (aberration) along that direction. I'm thinking of a setup like that in a point diffraction interferometer. $\endgroup$ Aug 15, 2015 at 0:34
  • $\begingroup$ @Joseph Don't worry about point diffraction interferometer too much: I just thought if it were wonted to you that it would be a way of explaining what the first slit does as the principle is the same. A subresolvable pinhole destroys wavefront phase information, and so is a way of getting aberration free light. Naturally, the process is very lossy, hence the first lens to try to mitigate the loss as much as you can. $\endgroup$ Aug 15, 2015 at 1:16
  • $\begingroup$ @Joseph Optical physicists seem not to be too good at explaining the basic physics of their setups. It is hard to get definitive references and really you simply need to set up a system calculation to check powers, image sizes and so forth. $\endgroup$ Aug 15, 2015 at 2:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.