Date: Sat, 12 Feb 2011 15:20:02

Author: Ann Reagan

Subject: Re: Question Speed of Light Experiment

Post:

We also do the chopped laser/oscilloscope experiment to measure the speed of light. It is a direct measurement of distance and time, and makes the connection to speed very apparent. It is also often the student's first practical use of an oscilloscope.

For comparison, you might consider two other approaches:

1). EM Theory: Use the current balance experiment to determine permeability of free space. Then, use the capacitive plate attachments to determine permittivity of free space. Inverse of the sq rt of the product = c.

2) From an e-mail ad from Arbor Scientific: Stick a large, long chocolate bar in your microwave (with no rotating platform). Heat just long enough for uneven melting to begin. Distance between melted stripes is distance between wave antinodes (i.e., half a wavelength). Multiple wavelength by frequency of the microwave, usually 2.45 GHz, to get c.

I tried the second approach yesterday with mini-bits (the smallest sized chips for baking cookies) spread in a mono-layer on a ceramic dinner plate. 25 seconds in the microwave seemed to work well. The stripes of partially-melted chocolate were clearly visible, and after I measured the distance between them, I got to eat the leftovers. I also found a website for looking up the frequency of microwave ovens (in case, like me, you threw your owner's manual out years ago). Inside the door or on the back of the microwave, next to the model number, will be an FCC ID#, which you can enter at http://www.fcc.gov/oet/ea/fccid/ to find the frequency. Final result: c ~ 3.2 x 10^ 8 m/s.

Enjoy!

Ann Reagan

Dr. Ann M. Reagan