Category Archives: physics

Me at the NECC

I’ve been hurriedly getting my stuff together for the upcoming NECC. I’d be doing great as far as time goes if it weren’t for the fact that I’m also working with a team of students who are trying to finish their invention project. They won a Lemelson-MIT InvenTeams grant and will be presenting their project in Boston from June 19th thru the 23rd. I’ll be going with them, which will be cool, but I’m running out of time!!!

Any way, on with my stuff. I’ve been uploading directions to This site is pretty cool and they have a competition running right now where the best project wins a laser cutter. While I don’t think I’ll win the laser maybe they will send me a free T-Shirt. Although if I do somehow with the laser cutter the science club next year will have the best fund raisers ever.

Below you will find links to the projects I’ve submitted so far. I will have at least one or two more to put up yet before I’m ready for Atlanta.

I’ve been contemplating ways I can use Instructables in class. I’ve used some of the projects either for demos or student projects, but I haven’t had students upload their own yet. Their had been a problem with the Instructables web site and our network firewall, but this seems to have been resolved. Next year I’ll have to use this. I just have to figure out a good way that won’t swamp the site with 90 projects for the same thing.

CD Diffraction and quick fun bits for teaching light

Well, the FOCUS Workshop/DMAPT Meeting was Awesome!!! I learned lots, and have probably already forgotten some great stuff.

I presented CD Diffraction and there was a question about how to remove the label from the back of a CD. I’ve never been good at this, and said as much. Some people just have the knack while most don’t. One of the audience members came to my rescue. He is one of the editors of ArborScientific’s Cool Stuff Newsletter (which is great btw). In Newsletter 22 they covered CD Diffraction and the used packing tape to remove the label.

Apparently you can just wrap the tape, sticky side out, around you hand and then just press down on the label and the tape will peel it off. Brilliant!!! The added bonus is you aren’t left with little foil bits scattered around the classroom.

More Diffraction
Find the diameter of a hair.

  1. Point a laser pointer at a hair. You will see a diffraction pattern on the wall.
  2. How do you line it all up? Tape the hair to the laser so that it is draped over the front.
  3. Measure the distance to the screen and the distance from the laser spot to the bands.
  4. Plug the numbers into the same formula from the CD Spacing activity from my last post.

Model DNA X-Ray Diffraction.

  • Same instructions as above, but use a 2-ply thread instead of a hair.
  • The 2-ply thread is a double helix
  • Shine on a screen or piece of paper
  • You should see a pattern similar to the X-Ray Diffraction picture that led to the discovery of DNA’s structure.
  • The picture to the right is Rosalind Franklin’s picture that was used by Watson and Crick.

I’ll try to post more stuff from the Workshop over the next few days.

DMAPT Optics – CD Diffraction

The Detroit Metropolitan Area Physics Teachers will be meeting at the University of Michigan today (Feb. 3, 2007). We are being hosted by the FOCUS Center.

As a part of the meeting we, the physics teachers will be sharing our own tips, labs, and demos with the other teachers as well. I will be sharing a cool use for old CDs. You know, the ones you used for backups and don’t need anymore or ones that didn’t burn correctly.

CDs use a diffraction grating to align the laser with the data on the disk. This is why we see rainbows on the bottom of the disks. You can use this property to make a simple spectroscope. A spectroscope allows you to see the spectrum of light you’re looking at. You’ve probably seen a demonstration of a prism separating white light into a rainbow of colors. A spectroscope will do the same thing. The fun part is that all lights do not give off the same spectrum. You will see distinct bands rather than a continuous spectrum.

There are several links on the web showing you how to make a CD Spectroscope, my favorite is one by Alan Schwabacher at the University of Wisconsin-Milwaukee.

I’ve but a few of these together. In general the more time you spend the nicer it will look, but you only need about 5 minutes to make a functional spectroscope capable of illustrating the differences between incandescent and fluorescent lights.

Another fun thing to do with old CDs (or DVDs) is to experimentally determine to groove spacing. Diffraction Gratings work because they have a series of fine grooves, too small for us to see. CDs are not truly diffraction gratings as they have no grooves, but actually have lines of pits that effectively act as a grating. The colors are separated due to interference. At smaller angles the shorter wavelengths of light will constructively interfere so we see violets and blues while at higher angles we see constructive interference in the longer wavelengths.

In order to determine the groove spacing you need only an old CD a single point light source (light bulb) and a ruler. The whole activity is outlined in a guide put together by General Atomics. You’ll want the section on the Electromagnetic Spectrum, specifically lab 3b.

In the basic setup you want the light behind you. You hold the CD up and try to center the reflection of the light in the center of the CD (i.e. in the hole so you can’t see it). Then while keeping the reflection in the hole you slowly bring the CD closer to your eye until you see a circular rainbow extending around the whole CD (you have to get it pretty close to your face, so your head may obscure part of the light). You then simply measure the distance from the CD to your eye and the distance from the center of the CD to a band of color that you know the wavelength for. In the General Atomics activity they use 450 nm for violet. I will often use red (620 nm) as well. You then plug the data into the formula and you get your answer.

Another way you can do this activity is to peel the label off the disk. I’m not very good at this, but I know it can be done. You can then look through the CD at the light. Again, you center the light in the hole and bring the disk towards your face. Sometimes when you buy a stack of CDs they will include one with no label. The picture below was taken through one of these.

You can even see where there are some scratches on part of the disk. Interestingly the violet band does not show up in the picture. I could see it, but the ccd in my camera failed to capture it.


I’ve been enjoying many of the projects on Instructables from some time. Today I contributed my first project.

Instructables is a site for people to “share what they do and how they do it.” It is an incredible site. There are projects on nearly any subject you can think of including: cooking, electronics, robotics, catapult making, programming, paper airplane making… the list goes on and on. Anyone can see them, but if you create a login ID you can comment on the projects or ask questions of the project creators. A login is also required to submit your own projects.

The project submission process is no more complicated than writing a blog entry and there is the possibility to collaborate on projects through the site, although I haven’t tried this yet. Even if you don’t use the collaboration tools on the projects, the ability to leave and receive comments can lead to an informal collaboration. Within a few hours of posting my project it had already been commented on and I was able to elaborate a bit to make everything more clear.

My project deals with the creation of the solar cell probeware I’ve outlined previously. In the future I’ll be adding more instructables detailing how to use the probe and software (Audacity and Visual Analyser) for a variety of lab activities or teacher demonstrations.

I’m considering incorporating this site in future student building projects. I’ve already referenced it for my electronics students and some students searching for good science fair projects.

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Do solar cells react fast enough?

A friend asked me this question when I shared my idea of using one as a light probe. I replied by asking are they fast enough for what?

Here is my VCR remote control. The first track is channel up and the second is channel down. If you look close you can see differences in the patterns. The IR from the remote is easily picked up by the solar cell.

The second picture shows the fluorescent light in my basement and my computer monitor respectively. The monitor is set to 85 Hz, I was able to determine this in Audacity. Actually I got a value of 84.8 Hz, but I figure that’s close enough.

On to light – More sneaky uses for free software

We started light this week in my physics class. I do a set of labs using CBLs (from TI) with the TI83/84 graphing calculators. We look at bulb wattage vs. brightness and distance vs. intensity (finding an inverse square relationship). I also have students see the fluctuations in a regular light bulb just caused by alternating current.

For those peope out there who don’t have CBL’s, LabPro’s, or PASCO probeware all you need to do these labs is a cheap solar cell and a computer running Audacity and/or Visual Analyzer. You’ll also need a 3.5 mm headphone jack. You can pick this up at Radioshack or you can go to the dollar store and pick up a cheap set of headphones.

Headphones will typically be stereo so there will be two leads, one for each ear. You only need one ear’s worth of wire. When you strip the lead you will find two wires, solder one wire to each of the leads on the solar cell. Then simply plug it into the microphone jack on your computer and load up Visual Analyzer. Point the cell at a light and see the 60 Hz alternating current (shows up as the light turns on and off 120 times a second).

You can use this to determine relative intensity, but to get exact lumens or lux you’ll have to find a way to calibrate your probe. You’ll need a volt meter for this. You can find them for under $10 if you look around a bit (assuming you don’t have any already).

The solar cell will also pick up infrared light. Just point a remote control at the cell and record in Audacity, zoom in to see how the pattern is different for different buttons and different remotes.

For under $5 you can have a very versitle light probe suitable for a number of great labs or demos.

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Teaching Sound

I’m finally up to teaching sound in my physics classes. This week we did a number of sound related labs/activities. One was determining the speed of sound using a long tube and a flat surface.

The tube (1.5 – 2 m) is stood up on a hard surface (tile floor). Using Audacity you can record a snap at the top of the tube and the echo of the snap off the floor. It’s really quite cool. Once you zoom in quite a bit it looks something like this. I’ve written up fairly comprehensive instructions. If you have any questions or need further clarifications feel free to respond to this post.

Audacity Echo.pdf (Sorry, my link is broken. I’ll fix the link to the document soon. Just drop me an email and I’ll send you the directions)

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New Version of Visual Analyzer

I’ve talked about Visual Analyzer in the past as being a great free oscilloscope/function generator program. The author has recently released another new version. He has also created an English language website. The previous site was in Italian and used frames. Someone told me the Italian site doesn’t work if you use Safari (Macintosh web browser). The English site is a very simple site with no frames.

Visual Analyzer can be downloaded from:

New Stuff in the new version and/or new stuff I’ve discovered that may have been in the previous version:

  • “Peak Hold” in the spectrum window. There is a check box in the spectrum window that will capture the peak frequencies. This is really helpful for sounds that are of very shot durations.
  • X-Y Graph: You can do an X-Y graph of the right and left channels. It’s been too long since I’ve used an actual oscilloscope to remember why you might want to do this.
  • My desktop computer had problems with VA 7.0. I have an Intel motherboard with an integrated sound card. VA 7.0 did no like this and I was only able to use the microphone as my input device. VA 8 has resolved this problem.

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Visual Analyzer

My favorite free oscilloscope program just got better. I’ve mentioned it before and I’m sure I’ll be mentioning again some time in the future. Visual Analyzer is free and has been upgraded so if you’ve already got it you should probably download it again. The site is in Italian, just look in the top left corner of the browser window and click “Download VA 7.0.5”

The coolest new feature is the ability to create sounds composed of multiple waves of different frequencies and intensities. Why would you do this? Picture playing a sound from a musical instrument into the scope. Capture the different harmonics the instrument creates and then use VA to simulate the same sound. With each harmonic you add the sound made by the program should get closer and closer to the original.

I did have a problem with the new version though. There may be a conflict with my sound card. I’m going to run it by a computer nerd friend of mine to see if we can get it figured out. On my slightly older computer it ran without a hitch.


Sorry I didn’t write about my sound demos last week. The end of the school year is just too busy. Here’s the basic list:

I used the function generator to show interference, both constructive and destructive by setting each speaker to a slightly different frequency. You can create some really good beats. When you feed the signal in to Visual Analyzer (VA) you can see the beats as well.

I illustrated destructive interference by setting both speakers to the same frequency and pointing them at my microphone. I slowly changed the distance to the microphone of one of them until the o-scope showed nearly complete destructive interference. If you were ambitious you could use this to determine the wavelength of the sound as well.

Using VA’s function generator I generated a frequency tuned to a graduated cylinder to cause it to resonate. I used a cheap $0.50 speaker that was to quite to hear until it was brought near the graduate. You could use this to determine the speed of sound if you were so inclined.

I used a variety of musical instruments to show the different numbers and strengths of relative harmonics using the FFT in VA to illustrate why different instruments sound different even when playing the same note.