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# Scan10008 - Physics 7C minimum the result is a dark spot...

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Unformatted text preview: Physics 7C minimum, the result is a dark spot, even if the other effect by itself would not have given a minimum: zero times anything is zero. For instance, if you looked at some screen position and expected to see an interference maximum, but that position also happened to be where there was a diffraction minimum, then you would see a dark spot. Interference and diffraction maxima are sometimes called orders, and this phenomenon is known as a missing order. If the third interference maximum happens to be at the same place as the first diffraction minimum, making that position is dark, we say that “the third interference order is missing.” The total intensity pattern (interference and diffraction effects combined) is plotted in Giancoli. This pattern is called the spectrum of the diffraction grating. Notice that the separation of diffraction maxima is always larger than the separation of interference maxima. There’s a good reason for this, which you will figure out for prelab question 4. Because of this difference, we refer to the broad diffraction intensity shape as the diffraction envelope, and call the intensity maxima the intensity peaks (or just peaks) of the grating’s spectrum. In fact, because of this spacing difference it is more common to see a missing interference order due to a diffraction minimum than the other way around, which is why diffraction minima are more interesting than diffraction maxima. ll. How This Experiment Works We’re going to use our knowledge of interference and diffraction effects to measure the parameters of several gratings. Once we’ve detemiined these parameters, we’ll use a high-quality grating to measure a small difference in wavelength between two different types of lasers. Measuring wavelengths in this way is called grating spectroscopy, and is a powerful tool that we will use again in a later lab. For this experiment we need monochromatic light with a ﬂat wavefront, so we’ll use our diode laser 0:63SOA). The fact that laser light doesn’t diverge much tells us that its wavefront must be pretty ﬂat, since a curved wavefront would either converge or diverge. Most of our gratings are contained on a 3”x5” piece of glass, with different values of a, d and number of slits N. Figure 4 at the end of the lab shows a schematic drawing of the grating plate showing a, d and N for each grating. We also have a high-quality 2”x2” single grating with a very high value of N. We’ll use this for our final laser-comparison measurement. DON’T TOUCH THE CENTER OF THE HIGH-QUALITY GRATING. The slits are delicate and can be destroyed by ﬁngerprints, so handle it only by its edges. DIFFRACTION GRATINGS 8 48 ...
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