# Biology teacher says don't open microwave till beeps stop because microwaves still bouncing around **URL:** \ **Category:** Factual Questions\ **Created:** [February 12, 2016, 4:06pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943 "2016-02-12T16:06:34Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Left\_Hand\_of\_Dorkness](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/left_hand_of_dorkness/32/7156_2.png) [@Left\_Hand\_of\_Dorkness](https://boards.straightdope.com/u/Left_Hand_of_Dorkness)\ **Post date:** [February 15, 2016, 3:49am UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/41 "2016-02-15T03:49:34Z")
> [@wolfpup](#): > > I’m pretty sure it wouldn’t be safe to eat soup with photons in it. Soup is fine, but stay away from light beer. ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [February 15, 2016, 4:35am UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/42 "2016-02-15T04:35:25Z")
The biology teacher should teach his parents about the perils of inbreeding. ---
**Author:** ![iljitsch](https://avatars.discourse-cdn.com/v4/letter/i/77aa72/32.png) [@iljitsch](https://boards.straightdope.com/u/iljitsch)\ **Post date:** [February 15, 2016, 6:20am UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/43 "2016-02-15T06:20:49Z")
Although there’s no reason to worry about this, as I’m sure any concerns in this area would be mentioned in the five (!!!) pages of safety warnings in my microwave’s manual, there’s actually an interesting question underlying this. How long until the microwaves stop bouncing around? Let’s assume the microwave is 30 cm across. The speed of microwaves is 300,000 km/sec, so that allows them to bounce a billion times per second. How much energy do they lose on each bounce? Let’s say 0.01%. That means that after 0.0001 seconds, the 900 watts are reduced to 0.04 watts, the same as your wifi network uses (which happens to operate at the same frequency). Now hopefully there’s some food in there that actually absorbs more than 0.01% of the microwaves, so they stop bouncing even faster. Then again, the door sensor doesn’t actually engage until the door opens slightly, so the microwave may keep operating for a fraction of a second while the door isn’t completely sealed… ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [February 15, 2016, 5:43pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/44 "2016-02-15T17:43:48Z")
> [@jtur88](#): > > Yes, and if you turn off the light and you’re quick enough, you can be in bed before it gets dark… Actually, you don’t need to be quick at all. Just do it while the sun’s still up. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [February 15, 2016, 5:58pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/45 "2016-02-15T17:58:22Z")
> [@gnoitall](#): > > And also, just a few watts of RF energy. You probably get as much holding a cell phone to your head while talking. Yes, **exactly** , and that’s why they invented phones you don’t have to hold to your ear anymore. Prove me wrong! ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [February 15, 2016, 6:58pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/46 "2016-02-15T18:58:30Z")
> [@watchwolf49](#): > > Perhaps the proof you seek can be found using a Geiger counter … see, no readings. Wrong kind of radiation. ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [February 15, 2016, 7:29pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/47 "2016-02-15T19:29:41Z")
Question – does the magnetron stop emitting radiation the moment the power stops? You’ve got high voltage transformers and probably power supply capacitors, not to mention hot cathodes and collapsing electric fields. So I could certainly imagine that the microwave fall off isn’t a straight drop to zero, but rather a some kind of a curve. Granted, the fall off would probably be very swift, but most people are just stating that once the power goes out, radiation ceases immediately, and that isn’t even true for light bulbs and CRTs. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [February 15, 2016, 7:34pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/48 "2016-02-15T19:34:28Z")
No, it doesn’t stop instantly. That’s why you get a short blast of radio waves coming out of the unit if you yank the door open while it is running (not enough to hurt you though). The timer shuts off the magnetron and then beeps. By the time your brain has processed that it is hearing a beep, the magnetron’s output has dropped to zero. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [February 15, 2016, 7:40pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/49 "2016-02-15T19:40:31Z")
> [@Kinthalis](#): > > Only one of those things lets the government hear my thoughts! > > :: puts tin-foil hat on :: Uh, I think if the microwave oven continues to run after a catastrophic failure that putting a tin-foil hat (that is aluminum nowadays) is the last thing one should do. If you are close to the broken and running device, your scalp and hair will not like the burns because of the microwaves causing rapid heat and sparks in thin metal foils. ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [February 15, 2016, 8:26pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/50 "2016-02-15T20:26:54Z")
> [@engineer\_comp\_geek](#): > > No, it doesn’t stop instantly. That’s why you get a short blast of radio waves coming out of the unit if you yank the door open while it is running (not enough to hurt you though). > > The timer shuts off the magnetron and then beeps. By the time your brain has processed that it is hearing a beep, the magnetron’s output has dropped to zero. Enough to send weird signals to your radio telescope, if you have one. ---
**Author:** ![Hoopy\_Frood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hoopy_frood/32/8452_2.png) [@Hoopy\_Frood](https://boards.straightdope.com/u/Hoopy_Frood)\ **Post date:** [February 15, 2016, 10:14pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/51 "2016-02-15T22:14:33Z")
> [@iljitsch](#): > > That means that after 0.0001 seconds, the 900 watts are reduced to 0.04 watts, the same as your wifi network uses (which happens to operate at the same frequency). Huh. Didn’t know that. I guess that explains why sometimes when we’re streaming video through the WiFi, running the microwave will occasionally cause the streaming to just quit working and force us to restart the program we’re watching. ---
**Author:** ![iljitsch](https://avatars.discourse-cdn.com/v4/letter/i/77aa72/32.png) [@iljitsch](https://boards.straightdope.com/u/iljitsch)\ **Post date:** [February 15, 2016, 10:15pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/52 "2016-02-15T22:15:08Z")
Actually I believe the microwaves cut out pretty much immediately. I remember a wifi implementation that allowed for smaller packets so data could come through while the magnetron wasn’t transmitting any microwaves because the AC current was around zero as it goes from positive to negative or the other direction. ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [February 16, 2016, 12:42am UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/53 "2016-02-16T00:42:39Z")
> [@iljitsch](#): > > Actually I believe the microwaves cut out pretty much immediately. I remember a wifi implementation that allowed for smaller packets so data could come through while the magnetron wasn’t transmitting any microwaves because the AC current was around zero as it goes from positive to negative or the other direction. The microwaves do, but the magnetron doesn’t always. ---
**Author:** ![Pushkin](https://avatars.discourse-cdn.com/v4/letter/p/ee59a6/32.png) [@Pushkin](https://boards.straightdope.com/u/Pushkin)\ **Post date:** [February 16, 2016, 9:21am UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/54 "2016-02-16T09:21:46Z")
I remember something similar in school. Or physics teacher tut tutting people who don’t like the idea of irradiating food to preserve it, then our chemistry teacher in the lab across the corridor shuddering at the thought of this being done and us having to eat radioactive food. Possibly he’d never had a banana in his life. > [@bob\_2](#): > > This was a comment by a Biology Teacher… I am with Dr Sheldon Cooper on this: > > "_Oh, I see why you’re confused. No, her news sounded important, but what you’re forgetting is it was an achievement in the field of biology. That’s all about yucky, squishy things."_ [To XKCD with this Doctor Cooper.](https://xkcd.com/1520/) ---
**Author:** ![MacLir](https://avatars.discourse-cdn.com/v4/letter/m/9e8a1a/32.png) [@MacLir](https://boards.straightdope.com/u/MacLir)\ **Post date:** [February 16, 2016, 5:38pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/56 "2016-02-16T17:38:17Z")
> [@Senegoid](#): > > Actually, I don’t think a magnetron has any moving parts spinning around, other than the electrons flying through. It uses a magnet to pull the electrons into a curved path, past a bunch of resonant cavities, which resonate and emit photons, which go into the wave guide and then into your soup. > > [http://www.explainthatstuff.com/how-magnetrons-work.html](http://www.explainthatstuff.com/how-magnetrons-work.html) As I understand it, before they worked out how to put turntables in microwaves the waveguide had a fan-like unit to help “break up” the emissions and avoid hot spots. I cannot verify this, so you may take it with whatever quantity of salt you wish. :dubious: ---
**Author:** ![MacLir](https://avatars.discourse-cdn.com/v4/letter/m/9e8a1a/32.png) [@MacLir](https://boards.straightdope.com/u/MacLir)\ **Post date:** [February 16, 2016, 6:03pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/57 "2016-02-16T18:03:02Z")
> [@Pushkin](#): > > I remember something similar in school. Or physics teacher tut tutting people who don’t like the idea of irradiating food to preserve it, then our chemistry teacher in the lab across the corridor shuddering at the thought of this being done and us having to eat radioactive food. Possibly he’d never had a banana in his life. And the full truth being somewhere between. Some foods are routinely irradiated and no one notices, some lose quality if the irradiation is too much, and some can’t be irradiated at all. 🆒 ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [February 16, 2016, 6:29pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/58 "2016-02-16T18:29:17Z")
> [@Pushkin](#): > > Possibly he’d never had a banana in his life. Sounds like he _was_ a banana. ---
**Author:** ![Haldurson](https://avatars.discourse-cdn.com/v4/letter/h/51bf81/32.png) [@Haldurson](https://boards.straightdope.com/u/Haldurson)\ **Post date:** [February 16, 2016, 11:11pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/59 "2016-02-16T23:11:33Z")
> [@iljitsch](#): > > How long until the microwaves stop bouncing around? They will bounce around until they lose enough energy that they are no longer microwaves, or until they are absorbed by… something. Radiant energy (microwaves, light, heat, etc.) tends to scatter, as it bounces relatively randomly around the room. The problem isn’t that it keeps bouncing, possibly for an indeterminate amount of time. The problem is when its focused, before it gets scattered, or through a microwave element in the oven when its turned on. Just spending time in the sunlight is likely a lot more dangerous. ---
**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\ **Post date:** [February 24, 2016, 1:42pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/60 "2016-02-24T13:42:18Z")
> [@Crafter\_Man](#): > > Minor nitpick, but it’s actually 2.45 GHz. (Not that the difference really matters.) Maybe this explains why my laptop loses it’s WiFi connection when I run the microwave. This didn’t happen with my old microwave. Is the new one leaking? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [February 24, 2016, 2:01pm UTC](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943/61 "2016-02-24T14:01:32Z")
> [@Siam\_Sam](#): > > Similarly, you must be careful when opening your computer, lest all the data escape. You’re confusing data with the magic smoke. Never let that out. [Previous page](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943.md?page=2) [Next page](https://boards.straightdope.com/t/biology-teacher-says-dont-open-microwave-till-beeps-stop-because-microwaves-still-bouncing-around/745943.md?page=4)