Measuring the Speed of Light...With a Microwave

Today in our weekly staff meeting for the Physics 140 Discovery Room, we tried an experiment to measure the speed of light using a microwave oven. We had been hoping to use this as one of the demos for the week, but we got rather poor results. Specifically, we measured the speed of light at about twice what it should be. This experiment has been done successfully before by others, see here for one example, but since our attempt failed we were all sent home to think about where our error was.

We used the fax paper method since it would be the cheapest and make the least mess. (Like we really wanted 600 students handling chocolate bars or eggs in there all week.) Anyway, here's what you need:

• A microwave oven
• Thermal fax paper
• Paper Towel
• Pyrex dish
• Ruler

The theory is that since you know the frequency of the microwaves in your oven (2.45 GHz), you can determine the speed of light by simply measuring the wavelength of those waves. They're related like this

where c is the speed of light (which we're trying to get), f is the frequency of the microwave and lambda is the wavelength. Remember that microwaves are electromagnetic waves, just like light, and so they always travel at the speed of light.

So how do we measure the wavelength? Well, the way that a microwave cooks food is by setting up a standing wave inside the oven. Basically, microwaves are created by a device called a magnetron and shot into the main chamber. Then the waves reflect off of the metal walls and screen covering the door. This bouncing around sets up a well defined pattern of waves, called a standing wave. In one dimension, a standing wave just looks like a sine function,

In some places, the amplitude of the wave is large, meaning the electric field part of the wave is oscillating up and down at full strength, and in other places the amplitude is zero. The food in the microwave is cooked because the changing electric field forces the water molecules in the food to flip back and forth with the oscillation, heating the food. Conversely, the areas where there is no oscillation don't get heated. This is why your microwave rotates the food as it cooks, so that no part of the food is stuck in a place where it won't be cooked. The movie above shows one wavelength of a wave, the wavelength is the minimum distance it takes for the wave to repeat itself. Notice that the distance between the two peaks is half of a wavelength.

Our method for measuring the wavelength was to take some fax paper, which changes color from white to black when heated, and lay it over a moist paper towel on top of an upside down pyrex dish, like so

The only difference between what we did at the meeting and how I did it at home was that I made sure to press gently on the fax paper so that it was uniformly moist before starting. Then I just ran the microwave for about 15 seconds. The microwave heats the water and causes the fax paper to turn black. The idea is that we should be able to see a pattern of black spots where the microwaves have the highest amplitudes and that the distance between two spots will be half of the wavelength of the microwaves in the oven.

My first try produced this:

Two splotches, six centimeters apart. Which means that the wavelegth of the waves in the microwave is twelve centimeters, giving us a speed of light of

.

The accepted value is

.

So that's pretty good! At the meeting, we got a wavelength of around 20 cm, and I'm afraid I'm not really any closer to knowing why, unless we just didn't properly wet the fax paper.

My second run was even better:

You can actually see that there is a standing wave in both the horizontal and vertical directions (at least, that's what I think this is showing). There are three blobs in the vertical direction spaced about six centimeters from one another, and two in the horizontal.

Very cool.

-Tim

While nothing I did here was exceptionally dangerous, there is still some risk of damaging your microwave or setting fire to the paper if you tried this yourself. Please do not try this at home.

(If the images and movies load too slowly or not at all, try going here.)

Cheap Car Insurance Day

Happy 'Cheap Car Insurance Day' to me!

Tamara got me a new DVD shelf. It's really nice, but we're already thinking about having to get a second one soon! ;-)


-Tim

Lab Reconstruction Day 14

And on the two week mark (today) we were able to bring down system B. The picture here was taken from about the same place as the one in the day 9 post, and system B sits prominently in the foreground.

Now things are getting crowded, and there's still quite a bit left.

This is probably only a partial list, but here's everything that's coming down that I'm aware of:

MBE System D & System D's electronics console
The XPS System & electonics console/computer
The STM & console/computer?
Two 15' lengths of connection tubing
One short length of tube
Ion and cryo pumps for the tubes, along with electronics and controls
Clean work table/air filtration hood (it's HUGE, we have no idea how we're getting it down)
Ultra-pure de-ionized water filtration system
MBE System A's electronics console (going into storage as spare parts)
All of the various tools/spare parts we can grab

Somehow I don't think we'll be done by Thanksgiving like Jim was hoping.

-Tim

Lab Reconstruction Day 9

By day nine, we had successfully moved system E down to the lab. At this point, we're starting to make some noticeable progress up in the Epicenter, and the lab still looks pretty spacious in this shot.

At the end of last week we brought down the electronics console for system B (Soren and Mike's system). Additionally, a few more compressors, their cables, and computers came down. All of that is behind the camera in this shot, I'll post the reverse at some point.

-Tim

Exam Review

I guess the reviews of my Physics 140 lecture were good, as I was asked back to do the review for the third exam in class today. All I did was go over last semester's exam, as opposed to trying to condense the last month's worth of material into an hour long lecture, since that's what the students seemed to wanted. It went well, but it was a lot of talking...Although, at least this time I had a working microphone.

-Tim

Lab Reconstruction Day 7

Well, here's how things stand at the end of today. We did manage to get system F down last Thursday, but not that much else has really moved since then. Some of the compressors for cryopumps, another large power supply and some more random electonics, but nothing big. However, system E is scheduled to be moved tomorrow, if all goes well.

Mike asked me if I could do a slide show of the pics so far, and it actually looked kind of cool. I may have to put something together for the Physical Revue this year.

-Tim

Lab Reconstruction Day 2

And here is a shot from the end of today. So far we've been able to bring down the racks of electronics and power supplies for systems E and F, as well as the big 10 kV power supply for system F's electron guns and Paul's computer.

Notice the gigantic piles of cable on either side of the room. Each pile represents about a days work. The cable clutter in the back of each of the electronics racks was quite impressive--or ridiculous, depending on how you looked at it. Cables seemed to be bundled together almost at random, with other cables wrapped around the big bundles just to make it even more difficult to disentangle them. Several of the big power cables were far too long and double back on themselves multiple times, even causing some of the cable trays to collapse under the weight. Additionally, we've had to take out most of the ceiling panels in the Epi Center in order to pull all the cables out and disconnect everything since none of the cables in the ceiling were bundled and the caught on just about every possible obstacle.

We're hoping to move one of the actual MBE systems tomorrow, but we'll see.

-Tim

Lab Reconstruction

We began to move equipment out of the Epi Center yesterday, so I guess there's really no going back now.

I took a few pictures yesterday morning just so we'd have something to compare to once all of the dust settles. The floor cleaning crew had just been in the lab the night before and, as you can see, they did a great job. The electrical wiring and plumbing have also been upgraded to handle all the new systems coming in.

The system in the back of the first photo (west side of the lab) is the Oxide MBE system that Jim brought with him when he first came to UIUC. We moved the system about seven feet to the west to make more room. You can also see the two capped pipes coming out of the floor in the middle of the room. These pipes mark where the wall used to be.


Looking the other direction you can see Soren's bike! Which he still hasn't moved.

Our office used to be down at this end of the lab.

-Tim

Casting for the Pod People

The science journal Nature has been putting up its own experimental podcast. (what's a podcast?) I've listened to the last few episodes and it's pretty good.

I didn't think that I would like podcasts since I never got into talk radio (which is very similar really) but the Nature one is very short and to the point. They usually cover 4 or 5 stories in about 20 minutes, so there's no DJ chatter or other unnecessary fluff to get in the way.

I also liked NPR's 'Science Friday' but it's definitely a packaged radio show. There's more depth, but my attention span just isn't long enough for more than a few minutes of listening.

-Tim

Are you not entertained?

Tamara and I have been watching the show 'Battlestar Galactica' recently. (Ummm, the new one, not the 'classic', although it's easy to get confused since Richard Hatch is in both.) The show has actually been doing fairly well for being an 'original' series produced by the sci fi channel, but mostly it's just making me miss Firefly something fierce...to borrow a phrase.

The basic story in BSG is powerful and well established, the human race is responsible for its own near destruction and the survivors must band together in a long shot attempt to find salvation. Sure it's been done and it'll be done again, but they do it well here. The special effects and the 'gritty realism' style are interesting and much improved over the classic BSG. The level of technology and who/what happened to survive is a little less well thought out, but forgivable (they just happened to have one ship that's actually a giant simulated outdoor park, and it just happens to have airport style metal detectors for some reason?)

The characters in BSG are all well developed people with histories, strengths, flaws, complex relationships and varied motivations/purposes, which should make for interesting drama. But (and this is where my problem with the series begins) no matter how different the characters are, they all share one common trait...they ALL make the absolute DUMBEST decisions at EVERY opportunity. I mean it's literally ridiculous, these people are supposed to have millitary training but they're all incapable of simple logical thinking or handling any sort of emergency.

And the evil robot cylons that look like humans are just ridiculous. Are they robots or clones? The series hasn't committed yet. If they're robots, then they're made of metal, you can detect that. And even if they're clones, it's well established that they can communicate with the cylon home world, why can't the humans detect these signals!? Wouldn't they learn to be suspicous of anyone with a radio built into their skulls? Instead, the best anyone can come up with is a cylon test that takes eleven hours per person to complete? And why the heck would anyone trust that doctor with anything in the first place, he's obviously dangerously mentally ill! There are only 50,000 humans left, why are they not all accounted for and tracked? Yes that's a major privacy violation, but you're up against an enemy that looks like you and can wreak havoc without warning.

Grrr.

Bring back Firefly ...please. I promise never to complain that the pace is a little slow again.

Meanwhile, I'm trying to get Tamara into Mythbusters, now that's a good show.

-Tim