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.)