>> ALOM: This pattern of bright and dark lines, known
as fringes, demonstrates a key property of light:
that it behaves like a wave.
The pattern is formed on a screen,
but as I like to show my students, it exists all
the way back to this pair of vertical slits,
which is responsible for producing it. Now,
I'm using a commercially-available double
slit slide just held in a clamp, and behind that a laser also
held in the clamp. At A-level physics students will
be required to use this phenomenon, and the theory
which explains it, to calculate a value for the wavelength of
the laser light. Depending on which exam
board they're with, they'll either use the equation
lambda equals a x over D or lambda equals s w over D,
both of which relate the wavelength
to the slit spacing, the fringe separation and the distance
to the screen. If you plot a graph of the distance
to the screen against the fringe separation you should get
a straight line which has a gradient equal to
the slit spacing divided by the wavelength.
And if you know the slit spacing you can
calculate the wavelength. Now lasers are brilliant fun
to play with and it's inevitable that students will want to wave
them around and point them at each other. Now, none of us
likes to be killjoys but this practical does present
an opportunity to talk to students about working safely
and considerately in the lab. If you're thinking about buying
more lasers so you can do this as a class practical, you might
be tempted to buy the cheapest ones you can get hold of.
Cheap lasers might be marked with the safe one milliwatt
or less rating. However, they're often much much more
powerful than this, and really shouldn't be used by students.
We recommend that you only buy them from reputable suppliers.
If you've got any doubts, it's always worth
consulting with CLEAPSS.
I'm using a piece of graph paper stuck to a safety screen but
you can use a wall or whatever else is easiest. However,
it's best to avoid having lasers crisscrossing the lab.
Students should be able to use the graph paper to find
the fringe separation and it's worth getting them
to think about whether they should take their measurements
from the centre of the maxima or the centre of the minima.
It's also worth challenging them to think about how to get
the most precise measurement of that separation.
Commercial double slit slides can be quite expensive
and this experiment does take up a lot of space, so you might
want to just put one set of kit in the corner of the lab and get
students to come up one at a time to take their
measurements while the others get on with something else.
Hi Christina, I see you're showing off your
lateral thinking again.
>> CHRISTINA: Actually this is a different experiment.
So I'm using a diffraction grating here and it's a nice
compact arrangement if you haven't got very much
space per student in your lab.
>> ALOM: Very clever and it looks a lot safer because
the lasers pointing downwards.
>> CHRISTINA: Yeah that's true, but you must point out
to students that they might get stray reflections in this plane.
>> ALOM: So this works in a similar way
to a double slit. The physics
is slightly different, but I’d expect teachers to teach
it around the same time.
>> CHRISTINA: Yes, the relationship here is n
lambda equals d sine theta. So n is the number of the order,
the order of the fringe that you're getting.
Lambda is the wavelength. d is the spacing of the lines on
the diffraction grating, and theta is the angle between
the straight-through direction and the fringe
that you're looking at.
>> ALOM: But these are quite big angles so students can't get
away with using a small angle approximation for sine theta.
>> CHRISTINA: No, that's right. They should use trigonometry
to get theta. >> ALOM: Shall we give it a go?
>> CHRISTINA: So if we take two fringes either side
of the central maximum.
Hundred ninety one mm.
>> ALOM: But we’re going to divide that by two?
>> CHRISTINA: Yes, that's right. And then the height.
The distance to the screen. Taking it from there.
OK is that straight?
Two hundred and twenty five mm.
>> ALOM: How many lines on the grating?
>> CHRISTINA: 300 lines per millimetre.
>> ALOM: So if my calculations are correct that gives us
a value of 650 nanometres, which isn't bad for red light.
And if I was doing this with my students I'd get them to compare
their value with
the manufacturer's stated value.
>> CHRISTINA: Yes, and if you wanted to take
this further you could give your students a different diffraction
grating with a different number of lines per millimetre,
if you've got them, and challenge them to compare
the uncertainties they get for their values of wavelength.
>> ALOM: Brilliant. Thanks Christina.
When a version of this experiment was first
done by Thomas Young in 1803, it started a process
which overturned long-held beliefs about
the nature of light. It was a significant step towards
our modern-day understanding, and more than 200 years later
it’s still counterintuitive that you can shine light at a screen
and get darkness. This practical also shows how
a macroscopic measurement can allow us to determine
a microscopic quantity. It’s one of my favourites.
I hope you found that useful. In the description below you'll
find links to lots of other useful stuff like teachers’
notes and worksheets, and please don't forget
to subscribe so you can watch the other films in the series.
Okay this doesn't feel weird at all.