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:HOW MEASURING THE SPEED OF LIGHT WITH A MICROWAVE OVEN ?
AN ACCIDENTAL DISCOVERY
Like many of today's great inventions, the microwave oven was a by-product of another technology. It was
during a radar-related research project around 1946 that Dr. Percy Spencer, a self-taught engineer with the
Raytheon Corporation, noticed something very unusual. He was testing a new vacuum tube called a
magnetron, when he discovered that the candy bar in his pocket had melted. This intrigued Dr. Spencer, so
he tried another experiment. This time he placed some popcorn kernels near the tube and, perhaps standing a
little farther away, he watched with an inventive sparkle in his eye as the popcorn sputtered, cracked and
popped all over his lab.
Dr. Spencer fashioned a metal box with an opening into which he fed microwave power. The energy
entering the box was unable to escape, thereby creating a higher density electromagnetic field. When food
was placed in the box and microwave energy fed in, the temperature of the food rose very rapidly. Dr.
Spencer had invented what was to revolutionize cooking, and form the basis of a multimillion dollar
industry, the microwave oven.
http://www.microtechfactoryservice.com/history.html et http://www.sciencebuddies.org/
by-product
oven
self-taught
vacuum
candy bar
To melt
popcorn kernels
sous-produit
autodidacte
Grains de maïs
sparkle
To sputter
To crack
To pop
To feed
thereby
To rise
un éclair (éclat)
crépiter
se fendre
sauter
introduire
ainsi
augmenter
1) How has Dr. Spencer discovered the effect of the microwave?
2) Can we see the microwaves ?
3) For whitch wave lengths can we see electromagnetic waves ?
The goal of this experiment is to measure the speed of electromagnetic
waves in the microwave portion of the spectrum with grated cheese and a
ruler !
1) Remove the rotating platter and the drive mechanism of the oven.
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2) Make a line of grated cheese on a plate.
3) Measure the spacing between the cooked portions of the cheese.
4) Your goal in making this measurement is to find :
a- the average distance between the cooked portions
b- an estimate for the error of your measurement.
5) Look at the label on the back of the microwave to find
the frequency of the microwave radiation the oven
produces.
What is the frequency of the microwave ?
The equation that relates wavelength and frequency for electromagnetic waves is : λ.f =c where λ is the
wavelength, f is the frequency and c is the speed of light.
The spacing of the hot spots will be equal to one-half of the wavelength of the microwaves.
6) Calculate the speed of the microwaves using the wavelength (measured) and frequency (from the
oven label)
7) How closely does your calculation agree with published values for the speed of light?
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