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In this Veritasium video, a home experiment is presented which appears to produce a very good double-slit interference pattern with normal sunlight.

The experiment is an empty cardboard box with a visor and a placeholder for a microscope slide with two slits on one side. This is arranged with the slits and visor facing the Sun, so the interference forms on the bottom of the box.

enter image description here

They claim to observe a good interference pattern from the two slits:

enter image description here

Discussions of interference in optics textbooks often stress that coherent light is needed to produce such patterns, and that sunlight and other thermal sources of light do not have such coherence. How, then, is this possible?

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It's extremely time-consuming for other people to watch the whole video in order to find the relevant part. Please point us to the relevant portion where they show or explain what's inside the cardboard box. For example, one can make coherent light from sunlight by bringing light to a focus using a converging lens, and directing the bright spot to a single pinhole. Or one can use a double slit in which the distance between the slits is comparable to or less than the coherence length of the source; this is why diffraction gratings work with incoherent sources such as sunlight. – Ben Crowell Sep 8 '13 at 21:36

Yes coherent light is required. The important thing to realize is that coherent light is not something that is magically created by lasers. Sunlight is somewhat coherent and it's easy to make it as coherent as you like.

What do people mean when they say "coherent light"? Well, it can be a few different things, but the relevant criteria in this context are:

  • The light is all travelling more-or-less in the same direction ("spatial coherence" or "collimation")
  • The light is more-or-less the same frequency ("temporal coherence" or "monochromaticity")

(See Footnote.)

I say "more or less" to emphasize the fact that it is never 100% coherent, (even from a laser), and it is never 0% coherent (even from a lightbulb or sunlight)

The way to think about it is, the light travelling towards the double-slits coming from a certain direction (e.g. 10 degrees away from normal incidence) create a really nice sharp double-slit pattern. The light travelling towards the double-slits from a different direction (e.g. 20 degrees away from normal incidence) also creates a really nice sharp double-slit pattern, but shifted!

So if you have light coming from every direction between 10 degrees and 20 degrees, you see a blurry composite of all those different double-slit patterns. It's possible that it will be so blurry that you can't even see that there's any pattern there -- it's just blurred out into a smooth line. But it's also possible that it will be only a little bit blurred out and the pattern is still recognizable.

The reason there's a cardboard box in the youtube video is to ensure that all the light from the sky that makes it to the slit is travelling in more-or-less the same direction. (Do you see how that could be done? Take a cardboard box, poke a small hole in it, and then put a double-slit far from the hole ... all the light at the double-slit is now coming in the same direction, i.e. from the hole.)

Frequency (or wavelength) is basically the same: Different frequencies of light make different interference patters, and we see a blurry composite of all those different patterns at once. If more monochromatic light was used (e.g. red laser light), the pattern would be much less blurry and easier to see, especially far from the center of the pattern. Luckily we have color vision, so we can (to some extent) recognize the composite pattern for what it is -- we see rainbows near the center, not just a blur.


Footnote: In comments, people are complaining that the term "coherent light" should refer only to spatial coherence, not temporal coherence. I disagree: The term can refer to either of these, depending on context. For example, in the context of optical coherence tomography, or in the context of "coherence length", or in the context of Michelson interferometers, people routinely use the phrase "coherent light" to mean temporal coherence.

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This is not a correct definition of coherent light. Light of a single frequency is monochromatic. Coherence means that the phase is correlated over large distances, or maybe in pop-sci terms that the wave trains are fairly long. – Ben Crowell Sep 8 '13 at 22:17
When light is almost-monochromic, it's temporally coherent. When it's traveling more-or-lesss the same direction, it's spatially coherent. Do you agree? As I said, "coherence" means different things in different contexts, and there are certainly times when "coherence" specifically means only "spatial coherence" (as you propose). But I think I'm entitled to refer to temporal coherence as a kind of coherence. – Steve B Sep 8 '13 at 22:29
No, sorry, but this is just wrong. – Ben Crowell Sep 8 '13 at 22:40
I'm with @Ben on this. Monochromaticity and coherence are different things. Lasers are monochromatic (except the ultra-short pulsed ones) and coherent. Raw sunlight is neither. The light from a sodium vapor lamp is fairly monochromatic but not coherent. Light from a broadband source that has been passed through two pinholes in series is polychromatic and coherent. – dmckee Sep 8 '13 at 23:12
That said, in interferometric experiments (like the two slit job) you get results that are easy to interpret if you have a monochromatic source. Broadband sources will result in complicated multicolored figures on the projection screen which then require more careful analysis and explanation. – dmckee Sep 8 '13 at 23:17

Here is Young's original experiment with light ( after having studied water waves)

young dbslit

The first screen generates a point source, so as to create a coherent wave . If it is a pin hole the geometry assures that all the photons come from the same original tiny source of light. Nice illustration here , page 5. Coherent means that the phases describing the mathematical form of the wave are not randomized.

In the video above the slits must be narrow enough and the distance between them small enough so that the wavefront arriving at them is similar to a point source wavefront.In any case the interference pattern is sort of blurred due to the many frequencies.

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If the source is far away, light acquires a certain degree of coherence. Have a look at the Van Cittert–Zernike theorem, as pointed in wikipedia:

[...]the wavefront from an incoherent source will appear mostly coherent at large distances

The resulting fringes are different for different colors, but any color is maximum for straightforward direction. So, you see the bright spot at the center.

Then, the wavelengths our eyes are sensible to are not very different for this experiment. In other words, you may choose a distance between the slits such that wavelength/distance is the approx the same for all the frequencies your eye is sensible to (from red to blue), i.e. you choose a large distance. Then, the all frequencies between red and blue will approx peak at the same position. Blue will peak slightly before than red. From the figure, you indeed see the overlapping fringes given by the highest frequency you can see with your eyes (blue light) and the lowest (red light) soon afterwards.

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Newton and Young didn't have coherent light, they worked with white light and saw color fringes. But there is a second condition. The dimension of the light source has to be very small or the source has to be at a big distance (in relation to its dimension). In this cases the photons propagates parallel to each other and do not overlap each other on the observation screen.

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I have another perspective on it. Interference is a wave phenomenon which nobody knows why and how it happens. I am just putting my thoughts and see if it answer yours. When a photon reaches slit it needs to decide whether to continue as a photon or convert to a wave. Conditions which are favorable for conversion is wavelength and phase correlation of nearby photons. More cluttered the space with uncorrelated photons more difficult it will be for a photon to be a wave. Therefore a single photon will pass through but two photons with different polarization will not. I have not thought of a mathematical model but I think it can be built.

You can think of Sun as great source of light with good phase and wavelength correlation amongst the photons. Only thing needed is to collect photons and send them towards the slit. Hole does a fine job of producing an order amongst photons. If you just expose the slit to the Sun, it will clutter the space with unrelated photons and no interference will be observed.

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single photons build up the same interference pattern that a wave of photons does. – anna v Jun 20 '14 at 18:40

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