At night, I went outside. I had a box with two slits in it. I directed torch light towards it, but I saw only two bands of light on the wall and shadow from the rest of the box. Why did it not produce interference like a double slit experiment should?

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    $\begingroup$ From the headline I thought maybe you had thrown empty beercans at two gaps in a wall... $\endgroup$ – jjack Dec 12 '17 at 2:43
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    $\begingroup$ Take a more powerful torch (LED maybe), then close the aperture, then increase the number of slits (what about a comb actually). Never tried actually. Will try tonight in my bathroom with all doors closed and lights out. $\endgroup$ – Trilarion Dec 12 '17 at 8:24
  • $\begingroup$ I find its easiest achieved by using a cheap laser pointer. This ensures monochromacity and a well-defined phase relation of the individual photons. $\endgroup$ – stebu92 Dec 12 '17 at 12:34
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    $\begingroup$ Note: I have done this many times with a just laser pointer and a strand of hair. You do not actually need the double slits to see the effect, just a separation between the light that is a similar size to the wavelength itself. $\endgroup$ – BlackThorn Dec 13 '17 at 0:26
  • $\begingroup$ Probably the easiest way is to use a DVD for the slits, but this is multi (>2) slit experiment technically. And a laser. $\endgroup$ – Fizz Dec 13 '17 at 20:07

In order to see the interference fringes, four conditions must be fulfilled:

  • Your light source either has to be point-like or very far away from the slits,

  • Your light source must be monochromatic* (i.e. emit only at a single wavelength),

  • The slits must be very close together, and

  • The slits must be very thin.

Failing to meet any of these will generate enough noise to completely obscure any signal you hope to measure. Probably the first one to fix in your case is to switch from a torch (which is neither point-like nor monochromatic) to a laser (which is much closer to being both of those things).

*In order to see the usual interference fringes, you must use monochromatic light. There is a similar effect you can do with a point source of white light, though, as shown in the comments.

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    $\begingroup$ The interference pattern as it's name indicates is caused by interference between 2 waves with SAME wavelength. Indeed, the black part of the pattern corresponds to complete destructive superposition (this destructive superposition won't be complete if you have 2 different wavelength so you won't have a completely black part). While the lighter parts of the pattern corresponds to complete constructive superposition. Basically these to phenomenon will occur but will cancel each other with non monochromatic light so you won't have an interference pattern. $\endgroup$ – Gornemant Dec 11 '17 at 22:19
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    $\begingroup$ You can in fact see a pattern with white light, but it is not distinct bands of light and dark, it is banded regions of different colors. See for instance en.wikipedia.org/wiki/Interferometry#/media/… . That said the usual mistake people make on the first time out is making the slits too far apart—the usual figures are not to scale. Try taping two razor blades together and using them to cut a pair of slits in a piece of aluminum foil. $\endgroup$ – dmckee --- ex-moderator kitten Dec 11 '17 at 22:37
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    $\begingroup$ @HotLicks Actually, I don't seem to remember having that problem last time I did it. What frequency of vibrations would substantially affect the fringe pattern? $\endgroup$ – probably_someone Dec 12 '17 at 2:59
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    $\begingroup$ @dmckee See also this Veritasium video where he demonstrates with sunlight. $\endgroup$ – Arthur Dec 12 '17 at 7:59
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    $\begingroup$ "Failing to meet any of these will generate enough interference..." This is a bit unfortunately formulated. Generating interference was the goal actually. $\endgroup$ – Trilarion Dec 12 '17 at 8:22

The other answers are correct, you need a coherent, monochromatic light source to see interference fringes. This site gives detailed instructions on how to do it using a laser pointer and a nit comb:


You can buy everything you need on ebay. You will likely pay less for the laser pointer than the nit comb! (Make sure it really is a laser. When I searched I got lots of hits for LED cat toys. Not the same thing).

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    $\begingroup$ "you need a coherent, monochromatic light source to see interference fringes" ::pounds head on table:: The pattern you get from polychromatic light is more subtle than that due to monochromatic light but you can observe interference this way. There are two links in the comments to probably_someone's answer that show the results. $\endgroup$ – dmckee --- ex-moderator kitten Dec 12 '17 at 19:12
  • $\begingroup$ Is this the same experiment which, when you use electrons, turns 200 electrons into 400? This experiment demonstrates the wave nature of light. With electrons it demonstrated their wave nature. You just have to move the slits close enough if I recall. Can that be done in the bathroom? $\endgroup$ – Pedroski Dec 12 '17 at 23:39
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    $\begingroup$ @Pedroski: The wavelength of electrons at realistic energies (<1kV) is about a thousand times smaller. That makes them useful for electron microscopy, but not for bathroom experiments. $\endgroup$ – MSalters Dec 13 '17 at 14:14

First of all, you need a laser, which is a reliable source of coherent light. The second thing you should know, to get the interference pattern, the dimensions of the slits (or lets say apertures, they don't need to be slits) should be comparable to the wavelength of the light you are using.

Let's say you are using a good old red laser pointer, which has a wavelength about 632 nm (visible region is ~350-700 nm). You need to have apertures with dimensions about a few micrometers (same for other visible region colours). You could observe interference pattern with two tiny circular aperture or rectangular ones. You can use whatever shape you like, although the pattern you observe will have different shapes according to the different aperture shapes. What is important is that at least one dimension of the apertures should be about a few micro meters.

Note: You could also see the interference pattern for polychromatic light sources but it would be smudged as different colours of light will interfere on different locations on the screen.


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