Session 7: Electromagnetic Radiation

It's everywhere and can't be escaped. Articles have been written on it's dangers. Special glasses and creams are designed to protect you from it. But some leaks right through the walls of you house. Some passes right through you! What is it?

Electromagnetic radiation comes in many forms; most of you know if at light. But light is just one tiny part of the electromagnetic spectrum, the full range of radiation that exists. Your eyes are designed as very efficient detectors of light, but they can't see infrared radiation; but you can feel it with your hands---infrared radiation is detected as heat. The microwaves that heat your food are another form. And every house has another detector (or more) that detect radio and television waves, both types of electromagnetic radiation. Summer is coming and the commercials will be full of warnings about wearing sunglasses and using sun block lotion to protect against ultraviolet radiation, another kind that you can't see. When you go to the dentist, they might X-ray your teeth; that's another kind.

Electromagnetic radiation is all very much the same in some ways even though the kind of detector you need is very different and what you can do with it (or what it does to you) can also be very different. But all of them are "cousins" of the ordinary visible light you can see with your eyes.

Electromagnetic radiation is a kind of wave, much like waves on water. For a long time, scientists thought that just like water waves, light needed something to travel in. They called that something ether. No one had ever seen it, but they figured it had to exist because all waves travel through something. There was a lot of work done trying to find the ether. No one ever found it and after a while, some experiments were able to show that it didn't exist. That's one thing that makes electromagnetic waves different from other waves---they can travel in empty space. They are affected by matter (you hand can stop visible light!) but they don't require some special think like water for ocean waves or air for sound waves.

There are two ways of measuring waves: frequency and wavelength. The two are related like multiplication and division. In fact, if you multiply the frequency and wavelength of a wave, you get back its speed. For electromagnetic waves, that speed is the speed of light.

Powers of Ten

You know what a meter is and a centimeter. The microwaves in your microwave oven have a wavelength of about 12 centimeters. Personally, I find that confusing. I would have thought that a microwave would have a wavelength measured in micrometers. There are one million micrometers in one meter. I can't find any clear origin of the why they got called "micro" waves since they are only about 1,000 times smaller than AM radio waves, not 1,000,000 time smaller. I think the word micro simply got stuck on because they are much smaller, kind of like today it's popular to put the prefix "nano" in front of high-tech things to indicate "really, really small" not because it's really one billion times smaller (think of the problems if your iPod nano really were "nano" sized!).

AM radio waves have wavelengths from about 560 meters (corresponding to 535 on your AM dial) down to about 230 meters (corresponding to AM 1300). FM wavelengths are shorter, but still measured in meters. And TV are even shorter. Microwaves range from about 1 millimeter to about 30 centimeters. Below that range, the waves are called infrared, Infrared waves go down to about on micrometer, that is, one millionth of a meter. A little below that is were the deepest red light become visible to our eyes. With waves a little shorter than what our eyes can see you find ultraviolet. Shorter still are X-rays, and shorter yet are gamma rays. Waves shorter than that don't have names or just get called "high energy gamma rays."

So What?

Almost everything we know about astronomy comes from what we can "see" through electromagnetic radiation. The Hubble Space Telescope can "see" both infrared and visible There have been specialized space telescopes which can "see" radio waves, ultraviolet, or X-rays. Some of these telescopes can tell the difference between different wavelenghts of ultraviolet radiation just like our eyes can tell the difference between different colors of light. Those telescopes convert the different "colors" of ultraviolet radiation into colors our eyes can see. One of the bettern known such space telescopes is GALEX, the Galaxy Evolution Explorer which has produced a lot of pretty pictures.

Ultraviolet Detectors

Today we are making ultraviolet detector bracelets using special beads which are sensitive to ultraviolet light. In addition to turning bright colors when exposed to the Sun, you can use the beads to test your sunglasses. Put your finished bracelet behind your sunglasses and see if the beads turn colors. If they do, how much longer does it take?

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