Electromagnetic radiation questions

  1. According to my understanding, the division between radio and microwaves is arbitrary. Hence, the microwave oven was originally called the “radio oven” back in the 1940s. Is it true there is no real difference?

However, when we get to light, visible or otherwise, we find that infrared through visible to ultraviolet can be focused with lenses, etc., so the division between radio/light/x-rays would seem to have some practical validity.

  1. But what about the difference between x-rays and gamma? Is that arbitrary?

  2. Is there a maximum possible wavelenth for radio? Is there a minimum possible wavelength for gamma?

  3. Is there a maximum possible amplitude a wave can have? Also, I don’t get why amplitude can vary. I mean could you have a microwave, say, 1 mm long but with a 50 m amplitude?

  4. Why don’t microwaves fly out the transparent front door of the mike?

Thanks for your help.

All the divisions are at least somewhat arbitrary. Many of the divisions that would seem to have some merit - for example, not calling it “radio” if you can’t build an electronic circuit that runs fast enough to generate it - turn out to be arbitrary as technology changes what we can build or do.

“Light” can be focussed with lenses, but different lens materials work over different ranges. You can build a lens to focus microwaves, for instance.

The difference between x-rays and gamma is quite arbitrary if you are referring to the wavelength boundary between the two regimes of the EMR spectrum. But actually many consider that the difference between them is that x-rays come from an electron tube and gamma rays come from nuclear activity, so by looking at a photon you can’t say which it is - you have to know its history.

Whether there are maximum or minimum wavelengths for a category of EMR depends on your definitions. The maximum possible wavelength for any EMR might be defined as the length of the universe, and perhaps there is some maximum energy a photon could have, for example certainly one photon could not contain more energy than the entire universe.

Amplitudes of EMR are not measured in length, but in field strength. If you are picturing a drawing of a sine wave, remember that it’s intended as a graph and the vertical axis isn’t length, it’s field strength. I don’t know why there should be any limit to the maximum possible field strength (or “brightness”) that EMR could have, but there might be some limit like the “entire universe” reference above, I suppose.

Microwaves don’t fly out the front of the oven because it is not transparent, at least to microwaves. It can have a screen made of metal whose openings are much smaller than the microwaves (I thinik they’re about a centimeter long for typical microwave ovens). Maybe they also make semitransparent plated windows for these ovens, too - I’ve only ever seen screens, tho.

It’s all light, only the wavelength varies. And microwaves have always been recognised as radio waves, too. Although it’s more common now to talk about the radio spectrum in terms of Low Frequency (LF), High Frequency (HF), Very High Frequency(VHF), Ultra High Frequency (UHF), etc., it’s also possible to divide the spectrum into Long Wave, Medium Wave, Short Wave, Micro Wave, etc.

You can focus microwaves with wax lenses, too.

The distinction here is in the source of the radiation. X-Rays come from electrons, gamma rays come from nuclei.

There’s no minimum or maximum. Even if there were some minimum and maximum wavelength that could be produced, it’s always possible to red or blue shift the resulting light anyway.

This question doesn’t have any meaning. Light waves don’t have a physical amplitude measured in metres the way waves on the surface of water do.

Because the transparent door is coated with a metallic mesh or a punched metal sheet that reflects the microwaves.

Wow, that’s a lot of questions. And nobody has responded yet. The experts will probably have weighed in by the time I finish writing this, though, so my answers may be superseded before I even post them. But I shall attempt answers nevertheless!

I think that microwaves are a subdivision of radio waves, kind of like VHF, UHF, etc. are, but using wavelength as the classifying terminology instead of frequency. Not sure though.

And as for x-rays and gamma rays, I think there is a bit of overlap between their wavelengths, and we just call them x-rays if they have come from electron activity and gamma rays if they come from nuclear activity. So

And, you can focus other wavelengths of electromagnetic radiation with lenses, not just IR / visible light / UV. I once read that some microwave frequencies can be focused using lenses of wax or tree sap, for instance – but the relatively large wavelength means that a bigger lens is needed. The book had a picture of a guy standing next to an enormous black circle that was twice as tall as he was, which the caption said was a tree sap lens.


No, not that I’m aware of… definition-wise, some charts that I’ve seen have labeled the area after gamma rays as “cosmic rays,” but that always seemed confusing to me because “cosmic rays” usually refer to high-energy particles from space, not to an area of the electromagnetic spectrum.


Amplitude is just a representation of how “strong” the wave is – i.e., how bright it is. I don’t think that electromagnetic waves have an amplitude the same way a physical wave does. I believe that in particle terms, amplitude corresponds to the number of photons of that particular energy level (i.e. wavelength or frequency) that are being emitted by the source.


There was a good Staff Report on this just a month or two ago … ah, here it is:


There is no maximum possible wavelength for radio, but detection becomes tricky at the lowest energies. It turns out that radio/TV transimission signals are some of the longest radiowaves out there – many meters in length.

There is also no minimum possilble wavelength for gamma rays, but anything that is above 1.022 MeV or smaller than 10[sup]-12[/sup] meters has the interesting property that it can spontaneously create electron-positron pairs and interact with matter in that way. That said, we have detected photons of incredibly high energies (hundereds of GeV with wavelengths smaller than 10[sup]-17[/sup] meters) indirectly through particle showers.

Submarines use extremely low frequency radio waves for communications because they happen to bounce really well through the polar ice cap. I don’t recall the frequencies they use but they are under 1 KHz. That’s the lowest frequency (i.e. longest wavelength) stuff that I’m aware of in practical use.

If you are trying to send some sort of signal over radio, a radio wave has to be at least double the frequency you are trying to modulate onto it. In other words, if you are trying to modulate a voice signal onto a radio wave, your radio wave needs to be at least double the maximum frequency of the voice signal. Most of the energy in human voice signals is in the range of 1 KHz to 7 KHz, so if you figure a nice round 10 KHz to make sure it comes through clearly, you’ll need a radio frequency of at least 20 KHz.

I googled a bit trying to find a nice article on “electromagnetic spectrum” but most of the articles seperated “microwaves” and “radio waves” into two seperate catagories. As you said, the division is arbitrary. Your microwave oven uses a magnetron to generate its radio waves, which by the way is exactly the same device used to generate radio waves for radar (RADIO detection and ranging) sets and some radio transmitters back in world war II. Your microwave oven typically uses a frequency of around 2.4 GHz because of how water molecules happen to respond to those frequencies. Googling “cordless phone” will find you a 2.4 GHz phone in the first page of results. The only difference is that the radio waves used by the phones are low power enough that they won’t cook you. It’s kind of like sunlight. Normal sunlight won’t hurt you at all, but if you have it at a bit higher power, like through a magnifying glass, you can burn an ant to a crisp. The safety limit for how much power cell phones and cordless phones can transmit is based on making sure they can’t heat up any part of your head to dangerous levels.

Just in case your thoughts are headed in that direction, let’s save the whole cell phones = brain cancer stuff for a different thread (just search - the topic comes up here quite often).

You may have also noticed that computers are using clock speeds up in the microwave range. Circuits designed for these high of speeds actually require the designer to think more in terms of radio wave propogation and analog signal issues than the digital on and off type of design methods you would expect when designing a computer. For example, you’ll never see a PCI bus with more than three slots, because if you make the tracks on the motherboard longer than that then they will act more like antennas than tracks and most of your signal will be broadcast out into the air instead of going from one place to another on your motherboard. You think your computer has more than 3 PCI slots on one bus? Look closely. If you have more than 3 slots you will also be able to find a PCI bridge chip somewhere nearby. What you really have is two PCI busses, that just happen to be placed next to each other so it looks like one big one.

A related couple of questions:

first correct me if I’m wrong please… If I understand correctly everything in the electormagnetic spectrum (light, radio, gamma, etc rays) is electromagnetic radiation. In other words, it’s all the same stuff, except working at different frequencies. But also, they cannot all be created by the same process. You need a lightbulb to create light for example, but a magnetron to create radio/microwave (you couldn’t create light using a magnetron, right?).

Second question is: Can you CHANGE a particular wave to somehtign else? Can you increase the frequency of a light beam and turn it into an x-ray for example?

I’m wondering if perhaps you could generate light with small enough of a magnetron. The frequency of the wave a magnetron generates is inversly porportional to it’s size. To generate light you’d need a very small magnetron indeed if some other perhaps quantum effects dont get in the way first.

I know for a fact that you can change the frequency of light easily. I have a green laser pointer that uses an infrared laser diode and a frequency doubler crystal to get around the high cost of green laser diodes. Also, any time something is moving realtive to you, it’s frequency appears shifted. Move fast enough, it a light wave could be shifted to ultra-violet and hgher.

Actually, most physicists think that there is a “maximum energy” to a photon, namely the Planck energy, about 10[sup]28[/sup] electron Volts. At this scale, the quantum fluctuations of gravity become important, and it’s not unlikely that our familiar notions of space and time will break down.

I’d try to explain this better, but it’s late & I’m tired. Maybe tomorrow.

There are some processes that are very limited to emitting specific wavelengths (for example an electron-positron annihilation creates a certain, very short wavelength photon) and other processes like thermal radiation that emit a wide range of wavelenths. If you look at a graph of energy per delta wavelength on the vertical axis, by wavelength on the horizontal axis, and you make both axes logarithmic, then a blackbody radiator (such as a hole in a much bigger hollow container) will emit a curve that has a peak at a wavelength about equal to 0.001 / temperature where wavelength is in m and temperature is in K. The two sides of the curve will approach straight lines, the right side being less steep. Thus, there are nonzero values at all wavelengths (though usefully large ones in a narrow range.

Follow-up question.

If electromagnetic waves do not have an amplitude like that of a sine wave, what exactly does the wavelength/frequency mean? Does the photon jump that distance with each pulse?

An electromagnetic wave does have an amplitude like a sine wave. In fact it has two of them. One is electrical field strength, E, measured in volts/metre. The other is magnetic field strength measured in amperes/metre. Those two field strengths vary from zero to maximum to zero to -maximum to zero again, all in the space of one wavelength. That’s what wavelength means.

Your original error was in assuming that the amplitude was measured in metres. It isn’t.

And if you were wondering whether you can have two photons side-by-side having the same wavelength but different amplitudes, the answer is no. You can’t. The wavelength uniquely identifies the energy possessed by the photon, and so also uniquely identifies the amplitudes of the E and H fields.

In different media, say vacuum compared to glass, a photon’s wavelength and field strengths will vary. But two photons in the same medium having the same wavelength are indistinguishable. There can be no difference in amplitudes.

Let me see if I can take a shot at this.

frequency is related to time by the formula t=1/f
If you have an rf source of 20 khz (20,000 cycles per/second) than you will have a time/cycle of 1/20000=50^-6 seconds (50 microseconds)

The speed of light © is approximately 300,000,000 meters/second.
c * f =wavelength
300x10^6 * 50x10^-6=15x10^3 or 1.5 kilometer wavelength for a 20khz signal.

doing the same math but using a microwave frequency of 20 gigaherz (20x10^9) would give you a wavelength of 1.5 millimeters.

…and no the photon doesn’t jump that distance with each pulse, it propagates (travels) that much further with each cycle.

The best analogy of propagation that I know of is to observe the waves on the surface of a pool of still water when a stone is tossed into the pool.

I’d say the view that the Planck energy as a maximum energy for photons is a fringe idea as there’s no hard physics behind it and it’s been discussed before on this board.

The amplitude of a wave of light is basically it’s intensity, or in otherwords (assuming it’s coherent), the number of photons in a wave.

that would be c * t = wavelength

wavelength = c/f

15e3m is 15 km, not 1.5

20 GHz gives you 1.5 cm, not mm

(c = 30 GHz-cm is sometimes a useful way to remember it)

With respect to the relationship between a photon and an electromagnetic wave – anytime a photon is detected it presents itself as a particle, but in between observations, if it’s anything at all, it’s a wave. In fact the probability of finding a photon in a small volume centered on a given point in an electromagnetic wave is proportional to the square of the electric field vector at that point.

IOW it is usually postulated that light propagates not as a stream of photons but instead as a probability wave or, more commonly, as an electromagnetic wave.

On the low end, the maximum wavelength depends on the size of the transmitting antenna. It’s difficult to send out radio waves which are thousands of miles long (thousands of miles between one peak of the wave and the next.) You’d need an antenna which was thousands of miles long.

On the other hand, if you don’t care about efficiency, then you can make your antenna much shorter than the wavelength of the waves. Even a tiny coil can emit feeble radio waves at low frequency. Or if you crank up the electrcial power so the coil gets white hot, the emitted signal doesn’t have to be so feeble.

Connect a signal generator which switches sinusoidally between high and low every few hours, hook it to a coil, and it will emit radio waves with billions of miles wavelength.