In the film, Christina sets up the apparatus vertically, which both requires less space in the lab and reduces concerns over laser safety. however, the mathematics involved is a little more involved, and students must use trigonometry to find the diffraction angle \(\theta\).
The pattern obtained is easier to see, since the bright fringes (maxima) are well-defined ‘spots’. This is an alternative or additional method.
#Laser diffraction experiment discussion pdf#
We have a PDF version of the above diagram available for download. What are the uncertainties in the measured quantities, and how do we combine those to arrive at an uncertainty in \(\lambda\)?.Why do we want \(a\) (or \(s\)) to be as small as possible, and \(D\) as large as possible?.What would happen to the fringe spacing if we used a green or blue laser? Why?.Why is a laser a particularly suitable light source for this experiment?.\(D\) = distance from slits to screen, which should be as large as possible (ideally 2 m or more).Ī straightforward calculation can be done, or you can use the graphical method Alom describes in the film at 1’04”, plotting the fringe separation obtained for a range of different \(D\) values and using the gradient to calculate \(\lambda\). You can check the value you obtain against the value stated on the laser and see if it agrees, within your experimental uncertainty.Measure with a ruler or Vernier callipers, or use mm squares on graph paper as the screen. \(x\) = fringe separation, between adjacent maxima or minima.\(a\) = slit spacing, between centres – this information is probably printed on a double-slit slide.\(\lambda\) = wavelength of light (to be found).In practice the filter often costs more than a suitable laser, but if you’d like to explore the option, here’s a write-up at Practical Physics.Īs shown in the film, different exam boards and textbooks use different notation for the formula governing the diffraction of light using a double slit aperture: However, to get good results the source needs to be very bright, ideally narrow (collimated), and as close to monochromatic as possible. It is possible to avoid lasers by using a white light source with a coloured filter. But that’s a summary: read the CLEAPSS advice in full. Their recommendation is to purchase from an established office or IT supplier, and to ensure the specific unit you receive has a CE marking and a classification BS EN 60825 Class 2. Here’s their guidance on lasers (PDF link). Christina started what turned out to be an interesting discussion about how one might check this, on Talk Physics (registration required).Īs ever, one should check with CLEAPSS. Lasers can be sourced for very little money, but they’re often more powerful than the claimed “In many labs it’s hard to achieve a good blackout, and that carries its own classroom management issues: do you have appropriate places to store coats and bags, for example? What about trailing cables and other trip hazards, in a darkened environment?.
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