Experiment 13
The Wave Nature of the Electromagnetic Spectrum

General Introduction

If by some chance you are a little more interested in some of the "why"s of this lab, read the "Theoretical Ramblings" section (from the link just below). If not, that's fine; by now you should have read my thoughts on what the purpose of these labs is. In any case, it might be a good idea to take a look at the sections on what's important and what things to watch out for, as well as the hints section; both from a theoretical point of view and from a "how do we get this done right/quickly/at-all" point of view, they should hopefully help.

Theoretical Ramblings (feel free to skip this)

What's Important in this lab

• Light shows properties of waves in the way it travels; namely, it interferes with other light waves, producing a standing interference pattern.
• Light, or electromagnetic radiation, has the same properties and inherent characteristics no matter what its frequency; whether it's a meter-wavelength radio wave, a microwave, visible light, UV, its general behavior is the same.
• Lasers don't just look pretty, they can be useful, too.

Hints and Tips for making this lab a better experience

• Make sure to record the uncertainty associated with each type of measurement, along with some note about what gave rise to that uncertainty. You'll need it for doing the data analysis, and you need it to be complete...

Part I (of the lab)

• Make sure you only turn the screw of the microscope in one direction while you're measuring the slit separation. Why do we keep harping on this? Because these microscopes are older than I am, so there's a lot of backlash.
  What's backlash? Well, that's the extra "give" or "looseness" in the screw; you know, similar to how if you just turned the hot water tap up, you can turn it the other way a little bit before it catches and starts changing the water temperature. Once you get a screw going in one direction, you can stop and then keep going and it'll still read accurately, but don't turn it back the other way until you're done going in the first direction. So, start to one side of the slits and go straight over to the other side.
• Also, measure the slit width a couple of times, just to help prevent errors from things being not-quite-focused or whatever.
• Make sure to record down your uncertainty in how closely you can measure the various quantities, and a reason for each uncertainty estimate.
• As for sketching the fringes down to scale with each other, that's not the easiest thing to do (unless you're an art major, perhaps). What has worked in the past is taping or pinning a sheet of paper up onto the screen (make sure it's flat against the screen), and then use a pencil to basically draw the outlines of the fringes. If you do it this way, make sure not to move the screen, slide position, or laser, so that all your drawings are automatically in scale.

Part II

Part III

Part IV

• Using calipers (to measure the slit separation) is an art. The lab manual says to use them to "determine the separation of the centers of the double slit," but please don't try to hold up the calipers so one tip is at the center of the one slit and the other at the center of the other. You get no use out of the precision of the calipers that way, since the weakest link is your eyeing out what is "center"! Instead, here are a couple of good ways to make this measurement that use the calipers better and will give you much more reliable numbers:
  • Measure the distance between the left sides of both slits, then the distance between the right sides of both slits, and then average the two measurements (they should be similar, though).
  • (this is even better)
    Measure the distance between the inside edges of the two slits, using the calipers normally; then measure the distance between the outside edges of the two slits, using the "inside-measurement" set of tips on the calipers. Average these two measurements.
  Whichever way, it couldn't hurt to do the measurement a couple of times and/or measure at a couple of places on the slit.
• One way to tell if you don't have the slits quite perpendicular or centered is to notice that you don't have a maximum at 0 degrees. Ideally, when the slits are centered and perpendicular, there'll be a central maximum, just like with the laser. It's not critical for getting good results, but if you do set things carefully so that your central maximum is right on 0 then your graph will look nicer, and the numbers on one side should come close to matching those on the other side (notice I said should; don't rely on that).
• The lab manual says to take regular measurements every 5 degrees, but I would also, as I knew I was getting close to a maximum or minimum, move the receiver really slowly and write down also the angle at the maximum or minimum. This gives you some more data points and can help you in drawing a smooth curve on your graph.

Part V

Things to Watch Out For (safety-type issues)

• WARNING: Do Not Look At Laser With Remaining Eye.
• That's one of my favourite laser warning signs. But it's serious, and so am I. Yes, these are relatively low-power HeNe lasers; no, they're not much brighter than the ones they use at Wegmans; but yes, they can hurt you. Not easily, and not all that likely, but are you really willing to bet your sight on it?
  • The light from the laser, even though it's not much, is extremely well collimated (means all in one line in pretty much exactly the same direction). Much more so than any natural light ever gets. As it is it wouldn't be a problem; but you have a lens in your eye, that focuses images down onto your retina. It takes this small, collimated beam, and focuses the whole thing down onto one or two cone cells. That's way too much light for just a couple of receptor cells!
  • The laser scanners at the supermarkets are doing just that -- scanning -- so that they are always very quickly shining in all different directions (it helps it read the bar code in lots of different orientations). Also, it helps it be safe: if you're standing where the laser is shining and you look at it, it won't be shining on those same couple of cells for even a significant fraction of a second, so it won't have time to overpower or kill them. However, in the lab, the lasers stay pointed where we put them, and our eyes' reflexes are sometimes not fast enough. I should know, since I work with much higher-power lasers (many of which are invisible so you have to make sure you know where they're going).
  This may seem like paranoia or being extreme; fine. It probably is. I just don't want to have anyone have a problem. These lasers are small, and fun, and you shouldn't worry about them at all or be nervous around them. Just don't be stupid and try to stare in one or start shining them around willy-nilly (easiest way to give yourself a low grade for this lab in my section), and you'll be fine.
• The microwave experiment is pretty safe; just remember that what's coming out of the transmitter is in fact microwaves like in your microwave oven, just at a lower power. Still, you wouldn't want to spend an hour sitting there cooking your brain, eh? So don't plonk yourself right where one of the transmitters is pointing directly at you. They're designed pretty well to be directional, so just don't sit straight in front of one for a long time and you'll be fine. (Walking around them isn't a safety concern or I wouldn't be in the lab with you . . .)

Things to Do for Fun that Aren't related directly to the lab

• See if you can cover just one of the two slits, and see how the pattern changes. Can you explain why the incredible difference?