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Measuring the Speed of 'Light' with a Microwave Oven
Posted Date:
20 Apr 2008
Total Responses:
0
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Posted By: sharathsathisan
Member Level: Silver Posted Date: 20 Apr 2008
Revenue Score: 
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Microwaves, like light, are an example of electromagnetic waves. Electromagnetic waves can travel through the vacuum of interstellar space. They do not depend on an external medium—unlike a mechanical wave such as a sound wave which must travel through air, water, or some solid medium. Electromagnetic waves cover a huge range of frequencies, from high-frequency gamma rays and x-rays, to ultraviolet light, visible light, and infrared light, and on into microwaves and radio waves. As the frequency decreases, so does the energy. The wavelength of an electromagnetic wave is inversely proportional to its frequency. So waves with high frequency have short wavelengths, and waves with low frequency have long wavelengths.
Electromagnetic waves interact with materials in different ways, depending on the nature of the material and the frequency of the electromagnetic wave. Microwaves work well for cooking because their energy can be efficiently absorbed by molecules commonly found in food, including water, sugars, and fats. The absorbed microwave energy heats these molecules and cooks the food. As you can see in Figure 1 below, the range of microwave wavelengths is from 0.01 cm to 10 cm.
the visible spectrum
Figure 1. The electromagnetic spectrum. Electromagnetic radiation covers a huge range of wavelengths. Light (the part of the electromagnetic spectrum that we can detect with our eyes) is only a small portion of this range. X-rays, light, and microwaves are all examples of electromagnetic waves. (Illustration from Abrisa Glass & Coatings, 2005)
In this project you will take advantage of some physical properties of waves in order to estimate the speed of light. These properties are interference and the relationship between a wave's speed, its frequency, and its wavelength. Interference is what happens when multiple waves interact. For example, at the beach, the incoming waves from the ocean and the outgoing waves from the surf drawing back from the beach interfere with one another. When two wave crests coincide, they combine to make an even higher crest. When two wave troughs coincide, they combine to make an even lower trough. When a wave crest and a wave trough coincide, they tend to cancel each other out. Interference is the name physicists use for this kind of combination of waves.
In a microwave oven, interference occurs between waves that are reflected from the inside surfaces of the oven. The interference patterns can create "hot" and "cold" spots in the oven—areas where the microwave energy is higher or lower than average. This is why many microwave ovens have rotating platters to promote more even cooking of the food. In the experiment described below, you'll remove the rotating platter (if your oven has one) in order to see the effects of the interference pattern on your cooking. You'll cook an egg white just long enough for some parts of the egg to solidify, while the rest remains partially cooked. The egg white will cook fastest at the hot spots in the oven. The distance between the hot spots will be equal to half of the wavelength of the microwaves. You will be able to measure the distance between the hot spots by measuring the distance between the cooked portions of the egg.
So measurements from your cooking will give you the wavelength of the microwaves. With one more piece of information, the frequency of the waves, you will be able to calculate the speed of light. You should be able to find the frequency of the microwaves on a label on the back of the oven. The frequency (f), wavelength (?, and wave speed (v) are related by the equation: v = f?.
The Experimental Procedure section below has all the details for doing the experiment. Do your background research and then go cook some eggs and see what they can tell you about the speed of light.
Terms, Concepts and Questions to Start Background Research
To do this project, you should do research that enables you to understand the following terms and concepts:
* Waves
o Frequency
o Wavelength
o Speed
o Standing waves
o Interference
* Electromagnetic spectrum
* Microwaves
* Microwave oven
* Magnetron
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