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.