Mar. 18, 2011

Science Dad on the Speed of Light and Sound

by Vince Harriman

Click to enlarge images

Beckett preparing the marshmallows

This week we decided to put the measuring skills we learned last week to the test. Since we are at the beach for spring break, we decided to utilize our location for our science experiments. We wanted to measure two of the most difficult things to measure -- sound and light. We started with light, because even though it travels much, much faster than sound, it is actually easier to measure and calculate.

This is a classic experiment that uses wave properties to measure the speed of light. Light is a wave (most of the time -- don't worry, kids, you'll get to the particle soon enough) and behaves just like every other wave you have ever seen. Since we are at the beach for spring break, we spent several minutes watching and counting the ocean waves as they rolled in.

We came back to the house, and I pulled out a bag of marshmallows and pointed to the microwave. "What do you call this," I said, pointing to the microwave, "and say it slowly." Beckett looked at me like I was crazy, then got a big smile. "A micro-wave," he said, emphasizing the 'wave'. Exactly.

We buttered a microwave safe dish (trust me, Moms and Dads -- it may be a science experiment, but the clean up is the same) and arranged mini-marshmallows along the bottom as neatly and tightly as we could. We stood them all up on end and packed them in so that there would be as little 'deadspace' as possible. We got out some chocolate chips, then disabled the turntable, placed the pan inside, and turned the microwave on low for 60 seconds. We watched the marshmallows cook and after about 40 seconds we had a very nice pattern of peaks and valleys. We pulled the pan out of the microwave, quickly placed chocolate chips at the top of each peak, then let our experiment cool.

4 5/8 inches between nodes

The pan now had a bunch of hot spots surrounded by uncooked marshmallows. The microwave generator inside the microwave sends out a very regular pattern of waves -- and these waves have high points (nodes) and low points (anti-nodes) just like the waves we saw at the beach. The high points (nodes) represent the high point of energy while the low points indicate low energy. By measuring the distance between points we could find the wavelength -- 4 5/8 inches, which we converted to the metric system and got .11178 meters. Speed (velocity in this equation) is easy to calculate: V=(wavelength)x(frequency). To find the frequency, all we had to do was open the door of the microwave -- the frequency is listed on the safety panel: 2450 megahertz. A hertz (the word hertz is used for both singular and plural) means one cycle per second. A megahertz is a million cycles per second. So: V (speed) = (0.11783 meters) x (2450 x1,000,000)=288,694,267 meters/second. Scientists calculate the speed of light at 299,792,458 meters/second, giving us an accuracy over 96%. Not bad for a bag of marshmallows, some chocolate chips, and a tape measure!

Next, we decided to tackle the speed of sound, which although it travels much much slower than light, is a bit harder to measure. We were hoping to perform this experiment on the beach, since the speed of sound is usually given "at sea-level," but the sound of the waves breaking on shore was too loud. So we moved just off the beach (you can see the dune fence behind Beckett) and measured off a good distance. Beckett was armed with a hammer and I had a camera with a zoom. We both had walky-talkies.

After I was in place, with the camera set up on a stable surface, I radioed to Beckett to whack the metal pole. I was just far enough away to register the delay between what I was seeing and what I was hearing. I radioed Beckett and had him whack the pole several more times so that we could get an average measurement. We headed back to the lab (house) and loaded the video onto the computer so that we could slow it down and measure the delay. At half speed we could easily measure the delay -- we used a stop watch and measured the delay ten times, then took the average divided by two (remember we slowed the film down by half.) This gave us an average delay between seeing and hearing as .78 seconds. This calculation is even easier than the first -- we divide the distance we were apart (in our case I measured 876 feet from Beckett to where I was standing) by the delay (in this case .78 seconds.) So V = 876 feet/.78 seconds or 1152.6 feet/second. The speed of sound at sea level is usually given as 1129 feet/second, so we were off by 2%! Not bad for a six year old with a hammer, a stopwatch and a camera!

Parent's Guide: Make sure you butter the marshmallow pan! All microwaves are different and you can quickly over cook marshmallows into a big sticky mess that will take hours to clean. And don't forget to turn off the turntable. As for measuring sound waves and doing science with kids in general -- please be careful and think through your experiments before you do them with kids! It should be fun and easy enough that they can participate and understand it. I confess that I came up with the sound experiment on my own, and experimented with putting the stop watch in the camera view, knowing that depth of field would suffer. I abandoned that idea before Beckett became too frustrated and we tried again knowing we could use the computer's video software to view and time the delay. No one was more surprised than I was (thanks, Science Mom) to find out we can measure sound pretty accurately with fairly common and available technology. And get creative with the tools around you. We considered using the panic button on my car -- when you hit the panic button both the lights flash and the horn sounds. This would have made a fine and easily recordable combination. Just keep it fun and try a low stress practice run. Then repeat, repeat, repeat. Science rewards repetition. Finally, don't let those warm marshmallows go to waste. Plan ahead and have two cups of your favorite cereal to mix in and make marshmallow treats. That will get some young scientists' attention!


Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett - 6 and Rowan - 2 1/2 ask him ‘why’ approximately 6,558 times a day.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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