Baleful bouncing beams

A reader asks:

This friend of mine is deathly scared of opening microwaves before they have finished. For example, put something in for a minute, wait about 55 seconds, get impatient and just pull the door open. The microwave stops, and my friend thinks that it takes some time (few seconds) for all the radiation to disappear. So if i ever do this around him, he thinks he might well be losing his ability to reproduce.

Is this true? I would have thought not, but you never know.

His technique is to wait until the microwave fully finishes beeping before it is safe to open.


The radiation level inside a microwave oven will actually drop to zero pretty much instantaneously after the magnetron is powered down by the safety interlock on the door latch.

Why, one might ask, is this so?

The radiation really is bouncing around in there, after all, reflected by the metal walls and the mesh on the inside of the door (the holes in the door-mesh are much too small to let through the radiation, which has a wavelength of about 12.4 centimetres).

Well, here's an analogy for you. Microwave radiation has a much lower frequency than visible light; 12.4-cm microwaves have a wavelength about 165,000 times that of the reddest light humans can see. But both microwave radiation and visible light travel at the speed of light, 299,792,458 metres per second in vacuum and very marginally slower in air.

Think of the microwave, therefore, as a mirrored box with a light-bulb in one corner lighting it up. Light's bouncing off the walls of the box, just like microwave radiation inside an oven.

If you turn the light bulb off, how long do you think the box would stay lit up?

The reason why the box would go dark pretty much instantaneously, just like the microwave, is that there's no such thing as a perfect mirror, for either wavelength of radiation. Even telescope mirrors only reflect about 95% of the light that hits them. And lightspeed radiation will bounce off the mirrors many, many, MANY times per second. So even a very slight loss of intensity with each reflection will eat all of the radiation in almost no time.

Let's consider radiation bouncing back and forth in the longest axis of a large microwave oven - let's say a whole metre - and assume that 99.9% of it bounces back each time. It'd actually be considerably less, and normal microwave ovens are much smaller than this, but let's presume someone's made a carefully-tuned microwave oven designed to resonate for as long as possible.

(This test microwave is also empty. Obviously if there's food in there then it'll soak up microwaves too, like a non-reflective object would inside a mirror-box.)

In one second, the microwaves in this giant super-reflective oven would have bounced off a wall 299,792,458 times - because that's the speed of light in a vacuum in metres per second - if there wasn't any air in the oven. Since light moves marginally slower in air, light would only have bounced about 299,702,547 times in a second if there were mirrors on each end and air in the oven. Microwaves slow down slightly in air as well, but even less than light.

The first time the radiation bounced, it'd be down to 0.999 of its original intensity. After bounce two, it'd be 0.998. After ten bounces, 0.99. After fifty, 0.951. It's easy to figure this out - it's just the portion of the radiation that bounces off, in this case 0.999, to the power of the number of bounces. 0.999^50 equals 0.951.

As you can see, the intensity is dropping pretty fast, and will be almost zero after a lot fewer than 299.8 million bounces.

After one millionth of a second there would have been almost 300 bounces, and the intensity would be down to 0.74 of its original value. After ten millionths of a second, there would have been almost 2998 bounces, and the intensity would be 0.0498. After a hundred millionths of a second - one ten-thousandth - the intensity would be down to 0.000000000000094.

In a real microwave oven, smaller and with much higher reflection losses, the microwave intensity would actually be functionally zero after less than a millionth of a second, even if there's no food in there soaking up radiation.

So you'd need to open that door pretty darn fast to encounter any microwaves.

(It's possible to jam microwave oven interlocks, or very occasionally for the safety systems to just fail, giving you an oven that can run with the door open. This is indeed hazardous to your health, but not nearly as dangerous as you'd think. In some commercial kitchens, all of the microwaves have, in a huge violation of numerous safety regulations, had their interlocks defeated for faster operation. Hobbyists have done many exceedingly unwise microwave oven experiments, too. But those hobbyists, and people who work in those kitchens, don't seem to come down with ghastly maladies any more often than socio-economically similar people with far less microwave exposure. As long as you don't actually get cooked - you can rapidly lose your eyesight if microwaves cook your eyeballs, for instance - there doesn't seem to be much reason to worry about microwave exposure that's far, far above what you'll ever get from even a half-broken, leaky home microwave oven. This doesn't stop some people from worrying about tiny electromagnetic-radiation exposure, or dastardly microwave-leak conspiracies, or what microwaved food may be doing to their precious bodily fluids.)

21 Responses to “Baleful bouncing beams”

  1. Tsavo Says:

    Daniel, excellent post, as usual.

    I can tell you, however, that my teeth are so nimble that I am actually able to capture individual microwaves with them.

  2. Red October Says:

    I'm still not quite clear on the mechanism of injury that exposure to microwaves would produce... Am I correct in assuming that they would basically do to you what they do to the food -cause your water molecules to vibrate, thereby generating heat?
    On a related note, my father, a former Air Force radar controller, related a tale of using a portable "gap-filler" radar mounted on a large truck. The radar team was teaching a border patrol team how to operate the truck, and one dimwitted BP agent climbed up the ladder to investigate the roof whilst the antenna was spinning... meaning the very powerful radar set was powered up (in spite of much signage instructing against such an action). He was eventually convinced to visit a hospital. Also, at the large sites (SAGE sites in the Mid-Canada line) they would spell out messages on the fence using burned out flourescent tubes, which would self-illuminate when the sweep went by. Somewhere I have a Kodachrome slide of "MERRY X-MAS" spelled out in flouro tubes zip-tied to a cyclone fence.

  3. Daniel Rutter Says:

    Yes, the only generally accepted way in which microwaves can hurt you is by heating you up. Grisly deaths have occasionally occurred when someone's repairing an extra-large commercial microwave oven, or monstrous radar system, and it's been turned on. If someone offers you $1000 to stick your hand in a safety-interlock-less microwave oven and turn it on for three seconds, though, I for one would take that bet. For another $500, I'd do it with a wristwatch on.

    There are many theories, some reasonably scientific and some completely wacky, about how exposure to low-level microwave radiation - like that from cellphones or wireless networking gear, for instance - might hurt people. None of the theories really seem to hold up, though, and despite what you keep reading in the newspapers, epidemiological evidence to support the idea that there's a significant risk is weak at best.

  4. Red October Says:

    Thanks Dan. I had no reason to doubt that it was anything beyond what I expected but I had never had it spelled out. I think most of the fears over EM radiation spring from the fact that we fear what we don't understand. Most people don't understand radiation at all. I wonder how many would freak out if I were to show them my glowing Tritium key chain... because glowing = strong radiation...

  5. Major Malfunction Says:

    You think you've got it bad...

    I work with a national RF *ahem* "appliance" network. These are LF devices (~100 kHz), with a maximum power of a few Watts on a 50% duty cycle (pulsing signal. Off half the time.).

    In compliance with precautionary OH&S laws, they have a radiation warning sticker. Don't stand too close...

    I constantly receive calls from people asking or complaining about *dramatic pause* T3H D34DLY RADIATION! *gasp*

    Even worse, they go around and switch them all off. Which means I have to track down a responsible person (their boss) and explain why they have to be on!

    That usually requires a layman's lecture on their function, the difference between radioactive and radiation, comparisons to TV, walkie-talkies and the mobile phone in your pocket.

    Then, the system runs fine until the next shift... Or a new employee...

    Seriously, Dan... I work with this stoopid all day, and I come home to THIS?!? What kind of Russian 'Net Bride are you, anyway?!?

    For Peter's friend, I recommend this.

  6. Darien Says:

    Ah, the lightbulb analogy is a great one -- totally cleared it up for me. And let me know where to source these one-meter-wide microwaves; we could use a few of those at my restaurant. Or should I just send you a badly-spelled e-mail order for fifteen of them, to be delivered to my address in China? You know, where my pump is?

    People have a lot of wild ideas about microwaves. As I mentioned, I work at a restaurant, and it's owned and operated by some rather remarkably woo-woo luddite types. Organic this, acupuncture that, let's-rearrange-the-kitchen-so-the-Feng-Shui-is-better. You know the type. They firmly believe that microwaving food in plastic containers gives you cancer, on which subject I have to withstand a fair few lectures. You wouldn't believe the level of panic I had to defuse when one of the microwaves developed a crack in the plastic panel on the front -- now it's leaking deadly radiation and it'll turn us all into horrible cancer-ridden mutants!

    I'm pretty sure they've just seen too many movies and don't understand that there are other kinds of radiation besides the one that made all those grasshoppers get big that one time. It doesn't actually need to be a guess; one of them explained to me during the panic that she knew it would turn us all into a Gamma World random encounter because she'd been to Three Mile Island, and the flies there are the size of hamsters.

    Apparently they didn't pass the square-cube law in New Jersey.

  7. Stuart Says:

    If I had a 'friend' like that, they would have to be very 'talented' in some other area for me to tolerate their utter idiocy.

  8. RichVR Says:

    Sometimes I think I'll commit a crime the next time I have to explain the difference between ionising and non-ionising radiation to some clueless git.

  9. Stark Says:

    @ Rich - I find it much more fun to simply tell them what the typical background level is and then tell them how much radiation the sun hits them with everyday. Then again, I've been accused of being evil on more than one occasion. :)

    My current job lets me observe the occasional radiation mishap from an amusing distance. I work with the local Hazardous Materials Response team (in an IT role - not as a hazmat guy) and have some great photos of the team digging through an entire garbage truck load of waste looking for whatever set off the landfills rad meters. Dressed in level A suits (the big yellow moon suits you see in movies - those are level A's) in July in California - temperature ~37C. Digging in the garbage in shifts for about 11 hours. All to fin the handful of radioactive tissues some unwitting nuclear medicine patient threw in the garbage because their doctor didn't give them the guidelines on what not to toss in the garbage for a couple of weeks. We still call one of the guys on the team Radioactive Snot Man (we even gave him a cape for his superhero persona).

    Of course we've had some much less fun run-ins with nasty high-powered ionizing sources - that's gonna happen occasionally with a big university next door. Radioactive snot however, is a highly amusing incident. Especially if you're not the guy in the giant pile of garbage.

  10. Red October Says:

    Ah, HazMat... a buddy of mine is a HazMat technician... since American police wear blue uniforms and tend to respond first to any emergency call, the HazMat crews call them "Blue Canaries" i.e. "They're all milling around over there, I guess that spot's OK, but there's one on the floor overe there and the rest are keeping away from him. Guess that's where we start!".

  11. Chazzozz Says:

    @Stark - been there, done that, and in +30C temps, too. You've got pretty much the right idea. I'm always willing to let the overly keen new chums to suit up and get stuck into it. I'll be quite happy to stick with the 'unglamourous' support roles, thank you.

  12. Gareth Pye Says:

    The better question is how long does the magnetron take to stop. I'm not particularly up to speed with how they work, but in general I'd expect the more efficiently they work the harder it would be to turn them off quickly.

    But that would hide the most important fact, that the radiation isn't super dangerous in the first place.

    [The microwave-oven magnetron is a resonant device, so yes, it will continue to "ring" for a moment after being shut down. That moment will be extremely brief, though, essentially I think because of the very very high frequency at which it's resonating. A tuning fork tuned to, say, 440Hz, may ring for a long time, but a tuning fork tuned to 2,400,000,000Hz won't. -Dan]

  13. j Says:

    I remember reading a somewhat hilarious press release from LG a few years ago (my, but what a reliable marketing electronics manufacturer they are).

    The thrust was that they were listening to their consumers and their consumers were concerned that they were eating irradiated food from microwaves!

    LG's answer was to produce a special "radiation-free oven", using infrared lamps.

    Apparently infrared isn't radiation; I'd have thought that discovery alone would warrant a press release, but never mind.

    Basically, they were selling a glorified pie warmer to stupid people.

    The last line set me off the most though:

    "Now we can all enjoy food, safe in the knowledge that it is untouched by radiation!"

    ...because we're all grunting and eating raw mammoth in a darkened cave?

  14. ULOFB Says:

    I dont want to go on - but your post about Neal Asher, hes done soemthing that really makes me so angry. He's called Obama a 'Watermelon'.(You have to scroll down to the "Hansen does it again" part)

  15. RichVR Says:

    I don't know who Neal Asher is, I guess it's a local thing. Sorry.

    But now I was wondering. Would a watermellon explode if microwaved long enough? Or would it just split open? Would it depend upon the thickness of the rind vs the amount of water in the mellon? Or would the thick rind cook and basically protect the juicy inner pinkness?

    Of course heating anything with liquid will make it boil. But is the rind of a watermellon strong enough to hold in the pressure until it bursts in a spectacular way?

    This, of course, does not take into account the natural effects of arcing in fruit or vegetables, that we all normally experience. There are the "grape experiments" of course. But I was actually thinking of the onion pops they we already know. Or on the expanding air pocket side, the thick soup expolsions that we have all known and loved...

    Sorry. A bit of herbal indulgence has clouded my intellect, as it is.

    Where is the delete button? Oh, here it

  16. j Says:

    Hooray, I've been looking for a tenuous reason to post this:

    It's a dude exploding a watermelon with gas injected from a hunting knife.

    Asher is an ass. He's just trying to maintain his ass status.

  17. Changes Says:

    My mom thinks microwaves are Invisible Death. I could write about this, but since I already did elsewhere I'm just going to link to the relevant thread.

  18. Kiro Says:

    As if by some freaky coincidence the trailer for Wes Cravens remake of "The Last House on the Left" was put on today. It ends with a guy being strapped to a table (possibly paralysed) with his head in an open microwave, which then begins to steam (he has wet hair). Nasty way to go I imagine.

  19. FuzzyPlushroom Says:

    j, the Koreans also believe in fan death, so I'm not surprised LG made a "radiation-free microwave oven".

  20. Jono4174 Says:

    People working with live microwaves (EG telecommunication technicians) have to be careful about the effects of heating up their eyeballs. Eyeballs do not have the cooling benefit of direct blood flow.

  21. bmorey Says:

    This fear might be more widespread than you think. I worked with a chap a couple of decades ago who believed a microwave oven wasn't safe open after it went off. He put off getting one for a couple of years until he wised up. I used the light globe analogy on him: "So in your house after you switch the room light off it stays light enough to see?"

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