“ROLLERBLADING MEN INVITED VITRIOL UNTIL X GAMES.”  That's how you can remember the waves on the electromagnetic spectrum (radio, microwave, infrared, visible, ultraviolet, x-ray, gamma), in case you ever need to.  But then you'll need a mnemonic for the not-so-memorable mnemonic.


alethiophile said...


As random trivia, X-rays and gamma rays are actually pretty much the same wavelengths; the distinction is drawn between photons sourced by electrons jumping around, like most EM radiation (X-rays) and photons coming from nuclear processes (gamma rays).

Texan99 said...

I just spent some frustrating time hunting for information on this. Everyone agrees that we use X-rays to describe photons that originate from electron shells and gamma rays for photons that originate from nuclei. After that, they switch units like crazy so that it's hard to compare the two, but it seems that photons that come out of electron shells normally have a wavelength of between 10 and 0.1 nanometers, while photons that come out of nuclei normally have a wavelength of 0.1 nanometers or less. The write-ups were so inconsistent that I'm not sure how trustworthiness that last formulation is. Readers?

(I particularly liked the explanation that gamma rays are more dangerous because they're "radioactive." Hey, thanks, internet!)

E Hines said...

I particularly liked the explanation that gamma rays are more dangerous because they're "radioactive." Hey, thanks, internet!

Yeah, when I broke my hand in high school, the doc used a plaster cast, and said the heat would go away when the cast cooled down.

On the matter of wavelengths, if my high school physics serves me (it's been a couple of years...), the wavelengths are related to energy content--higher energy and shorter wavelengths go together. The energy content of an atom's nucleus is lots greater than the energy content of electrons just putzing around in "orbits" around the nucleus (even if the orbits are just hypothetical constructs. The location of an electron at any particular moment--assuming its wave form has even collapsed yet into a discrete thing--is just a probability until it's actually observed in some way. But that's another story.)

Thus, when an electron changes orbitals, photons of a certain energy are generated. When a nucleus falls apart, the energy released is bound up in photons of much higher energy. So the nuclear photons have shorter wavelengths than orbital-related photons.

Eric Hines

Texan99 said...

I heard a weatherman once say "It's going to be getting cooler as the temperature drops."

Also, I've heard flames are dangerous because they can cause a fire.

The sites I found on the subject of x-rays vs. gamma rays were aware that frequency, wavelength, and energy were related, so I guess they thought it was OK to switch from one unit to another and say things like x-rays have this kind of wavelength but gamma rays are different because they have this other kind of energy levels. The few that tried to compare wavelengths would express one in exometers and the other in powers of ten. This included government websites such as the EPA, not just Yahoo! Answers. It looked like authors were cobbling paragraphs together from multiple sources they didn't much understand.

james said...

There's a continuum, of course.

The frequency times the wavelength is the speed, and since we're typically talking about vacuum (or air, which is pretty close), that's the speed of light. So if you know the frequency you know the wavelength and vice versa.

The energy of a single photon is proportional to the frequency. That makes sense--the faster you whip something back and forth the more energy it has.

What whips back and forth in a photon are the electric and magnetic fields. The magnetic field is largest when the electric field is changing fastest, and vice versa. (There are lots of nice symmetries in this business.)

You can think of starting with radio waves a few meters long. The energy of such a photon is tiny, and it is apt to just lazily bounce off or go around you. It acts a lot like the kind of waves we think about because its wavelength is large compared to the things we're used to.

Crank up the energy a bit to microwave wavelengths, and the back-and-forth is now fast enough to start shaking water molecules around a bit--so these photons can dump their energy into molecular motion (heat).

Crank up the energy a bit more to the visible light end of things, and we're into the realm where you can excite molecules into new states.

Crank it up a bit more, and the photons can cause changes in molecules. Sunburn, anybody? We don't use UV much for making chemical reactions go, but I suppose we could.

Crank it up a bit more into the soft X-rays, and you can excite some of the highest energy atomic states. In fact the reverse is the way we usually make x-rays: bombard metals with and electron beam to kick loose electrons from the atoms, and when the escapees are recaptured photons are emitted.

We're into a regime where the energy dumped from a photon may not be in one single place. An X-ray may kick loose an electron here (losing some of its own energy in the process), and another there, and another there. If a protein of yours happens to rely on those lost electrons, it may not work right anymore.

The harder the X-ray (the more energetic), the longer a path it is likely to take going through you.

Photons are more likely to interact with (and lose energy near) nuclei with larger charge--so copper or lead makes a better shield than calcium, and bones block more x-rays than muscle.

The harder X-rays can also match nuclear energy state changes, so they can be emitted (or absorbed) but nuclei.

Gamma rays have even more energy than the highest energy atomic states: you get them from nuclear reactions or even more energetic interactions. They are even more penetrating than X-rays, and of course dump even more energy.

In my field we tend to just say "gamma" no matter what the energy.