Tales from the Prep Room: Diffraction

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Experimenting with diffraction patterns.

In the second of his Tales from the Prep Room, Ri Demo Technician Andrew Marmery uses wire and a laser pen to recreate the famous cross-shaped diffraction patterns observed by Rosalind Franklin in 1952.

The biophysicist captured 'Photograph 51' of DNA whilst working at Kings College London. The photo – which revealed the structure of DNA – was later used by James D. Watson and Francis Crick as the basis for their famous model of the double helix.

By shining a laser through different configurations of wire, Andy is able to change the resulting diffraction pattern. Theoretically, he could then work backwards from each pattern to deduce the original position of each wire.

It is this idea that forms the basis of X-ray crystallography, using x-rays instead of laser light, and atoms instead of wires. The process allowed Rosalind Franklin to determine the 3D structure of DNA by analysing the X-ray diffraction patterns of crystals made up of the molecule.


Materials, Space & Time


Andrew Marmery
London, UK
Filmed in:
The Prep Room


Collections with this video:
Tales from the Prep Room, The Crystallography Collection

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  • Multiple wire diffraction set-up in the Prep Room

    Image: The Royal Institution
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  • Multiple-wire diffraction target detail

    Image: The Royal Institution
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  • Andy lines up the laser on the multiple-wire target.

    Image: The Royal Institution
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  • Andy makes incredibly precise adjustments to the laser. Shortly before dropping something.

    Image: The Royal Institution
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  • The diffraction pattern produced by multiple wires, randomly stuck in the beam.

    Image: The Royal Institution
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  • The diffraction pattern produced by a coiled-wire target - a lightbulb filament.

    Image: The Royal Institution
    cc_by-nc-sa License: Creative Commons

  • Not the surface of the sun with a green filter: a massively-defocused photograph of a diffraction pattern. Explanations welcome, particularly if they reference spatial coherence.

    Image: The Royal Institution
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  • Another view of the diffraction pattern produced by a coiled wire. High-res versions of these stills can be found on Flickr.

    Image: The Royal Institution
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So today we're going to try and do some laser diffraction, hopefully with a view, ultimately, to creating the famous cross-shaped diffraction pattern from the helix that Watson and Crick observed when they were doing their X-ray diffraction of DNA.

First mistake of the film, it wasn't, of course, Crick and Watson who did the X-ray diffraction work on DNA. It was...

Rosalind Franklin!

So first I need to go and get the laser, obviously. I thought I had one in here, but it turns out I don't.

I have no idea when this might have last been used.

So there's definitely something going on there when I pull the wire into the beam.

OK, so this looks a bit more convincing, if you like. We've got something interesting happening. Pretty definitely a diffraction pattern going on there. We've got some nice, dark fringes and light fringes. A bit of a mess in the middle, but it's spreading all the way over here. There is definitely something going on.

So what's happening is that the laser beam is effectively being cut into two by the wire. The wire is narrower than the beams, so the beam is passing around both sides. And you can see all the nice little, the nice pattern of, er... The waves add up and you get a bright... So anyway, they get to the screen. And because they both traveled, erm, not the same amount of, not the same distance... Anyway.

Don't know what you just did, but it was good, yeah.

OK, so don't mind this little light exclusion apparatus we just had to set up here. We can't really block out the lights in the Prep Room at the moment so we're having to improvise.

But you can see so much more structure now, obviously, because of the slightly crazy assortment of wires that we've set up. But we've got diffraction going off all over the place. And if you really look into the structure of the pattern, you can see this really fine structure throughout the two-dimensional field.

But you can imagine, that if we went through the whole process backwards, you could work out from this pattern where all those wires are. Because if you move any one of those wires then the diffraction pattern is going to change. So if you could do the math, then you could work out exactly where all those wires are from this pattern.

Imagine then doing that, not with laser light, but with X-rays, and, instead, not with wires but with atoms. And that is the basis of X-ray crystallography.

Another day, another diffraction target.

I'm going to remove the glass as carefully as I can.

That will probably do.

So I think we've got, well we've got a diffraction pattern there. It's a very, sort of, flat cross but the axis of the cross are very close together. But I think that's what you'd expect from a coil like this because it's a very tight coil. If you remember the diffraction pattern we got from the vertical wires were spread out into this horizontal pattern.

Basically we've got that happening from the coil from two different, but not quite perfectly aligned, sets of the vertical wires. We've got these, sort of, line on the near side of the coil, sort of, all lined up together. And then on the far side of the coil we've got the same thing but shifted very slightly as the coil, sort of, winds itself around.

So I think this is what I would have expected to happen. And if we, using my surgical tweezers, if we just sort of tease the filament out a bit. Hopefully as we extend the coil and we accentuate the difference between the, sort of, front and back of the coil, we should see the cross open out a bit. And yeah, it's kind of doing that. Probably only so far I can pull this before it breaks. But, yeah, that's quite nice.

So we've got this really lovely diffraction pattern just from our light bulb filament over there. And this is very characteristic of the diffraction you get from a helix or a coil or something like that. And it's what Rosalind Franklin saw when she was doing her X-ray diffraction of DNA. And it really influenced Crick and Watson in determining the structure and finding this famous double helix that is DNA.

And we've produced something pretty similar here with a light bulb and a laser pen. And I think it's pretty good. I'm quite pleased with it.

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