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-   -   Why are microwaves reflected by the door? (http://www.physicsforums.com/showthread.php?t=666774)

Emissive Jan25-13 04:45 AM

Why are microwaves reflected by the door?
 
Hi,

I am wondering why microwaves don't escape through the door?

The most common answer seems to be that the wavelength of microwave ovens is about 10-15cm which is much larger than the 1-2mm holes in the grille. But isn't the wavelength of light really just within a vector space of mathematics describing its motion not a description of the actual particle which remains point like? (Well - point like if you could find it in the probability density). We are not really looking at waves in reality as the have no amplitude - making a light wave from front view always look like a dot - and dots would fit through the holes in microwave...wouldn't they?

Bill_K Jan25-13 05:54 AM

Re: Why are microwaves reflected by the door?
 
Photons are not a universal substitute for classical electromagnetism. Both descriptions are valid and suitable, each in its own restricted domain. Photons are meaningful when the fields are weak and the energies high. This is not the case in a microwave oven, where we still use electric and magnetic fields obeying Maxwell's Equations.

The fields are confined to the interior of a microwave oven by a metallic mesh (size about 1 mm) that covers the walls and door and reflects the microwaves.

Emissive Jan25-13 06:26 AM

Re: Why are microwaves reflected by the door?
 
Yes....I guess electromagnetism does explain it but I hoping for the QED interpretation?

Can we imagine a single microwave photon fired at a 1mm hole in the mesh - why would it bounce back?

Vanadium 50 Jan25-13 01:20 PM

Re: Why are microwaves reflected by the door?
 
Do you already understand QED? If not, why do you want an explanation based on something you don't understand?

Emissive Jan25-13 02:22 PM

Re: Why are microwaves reflected by the door?
 
Well I wouldn't ask the question if knew the answer? :)

The part I am obviously not understanding is the dimensionallity of the photon. Is there a relationship between wavelength and the chance it will make it through a small opening?

Bill_K Jan25-13 02:51 PM

Re: Why are microwaves reflected by the door?
 
No, this is the part you are not understanding:
Quote:

Photons are not a universal substitute for classical electromagnetism. Both descriptions are valid and suitable, each in its own restricted domain. Photons are meaningful when the fields are weak and the energies high. This is NOT THE CASE in a microwave oven, where we still use electric and magnetic fields obeying Maxwell's Equations.

jtbell Jan25-13 03:18 PM

Re: Why are microwaves reflected by the door?
 
The material that the screen is made of makes a crucial difference here. A cardboard screen with the same size holes wouldn't block the microwaves at all, or only minimally.

sophiecentaur Jan25-13 03:50 PM

Re: Why are microwaves reflected by the door?
 
It's really unfair of photons not to behave like little bullets, isn't it? Life would be so much simpler if they were - and we wouldn't have to worry about diffraction and all the rest of modern Physics.
This OP is so much the product of really naff Science teaching and the poor level of presentation of Science on the Media. Why don't people appreciate that Science is Really Hard and not very approachable at all?

Emissive Jan25-13 04:53 PM

Re: Why are microwaves reflected by the door?
 
Yes - the emisivity of screen within the wavelength concerned certainly has major impact. The is why we wrap our buildings in foil to reflect some of the infra-red radiation and apply low-e coatings to glass to assist with heat loss/gain. Cardboard = no chance (virtually) metal = high chance of reflecting......microwaves in this case.

Ok - I think I worked this out by drawing it. Please correct me if I wrong...

A series of photons approach the screen. Their wavelength is 10x the size of the screen openings and the screen is 30% open. 30% of The photons do pass through the openings (the others hit the screen solid and reflect back) but they pass through every opening and arrive at a point on the side with varying amplitudes which then deconstructively build giving a very limited probability that they will be found there.

When the wavelength is changed to smaller than the opening we get a higher probability amplitude directly in front of the opening and deconstructive ones at holes further away.

Drakkith Jan25-13 05:06 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by Emissive (Post 4242861)

A series of photons approach the screen.

You are already wrong. Photons do not work like this. Instead, it is an EM wave of a certain energy and frequency that is emitted and propagates towards the screen. When the wave interacts with something, and by that I mean when it actually transfers energy, it will do so in little discrete amounts that we label as photons.

What sophie is trying to get you to understand, and what he has pounded into my head previously, is that you cannot say "a photon approaches the screen" or "what is the size of a photon". A photon is simply not a little particle that would behave in this manner.

sophiecentaur Jan25-13 05:14 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by Drakkith (Post 4242881)
You are already wrong. Photons do not work like this. Instead, it is an EM wave of a certain energy and frequency that is emitted and propagates towards the screen. When the wave interacts with something, and by that I mean when it actually transfers energy, it will do so in little discrete amounts that we label as photons.

What sophie is trying to get you to understand, and what he has pounded into my head previously, is that you cannot say "a photon approaches the screen" or "what is the size of a photon". A photon is simply not a little particle that would behave in this manner.

"But other fell into good ground, and brought forth fruit, some an hundredfold, some sixtyfold, some thirtyfold."

You have no idea how good that makes me feel! :smile:

Drakkith Jan25-13 05:25 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by sophiecentaur (Post 4242890)
"But other fell into good ground, and brought forth fruit, some an hundredfold, some sixtyfold, some thirtyfold."

You have no idea how good that makes me feel! :smile:

As hard as it was to wrap my head around initially, everything involving light makes a LOT more sense now.

sophiecentaur Jan25-13 05:27 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by Drakkith (Post 4242912)
As hard as it was to wrap my head around initially, everything involving light makes a LOT more sense now.

One has to chuck out an immense amount of baggage to get there. Most of it came from 'Education', too.

Drakkith Jan25-13 05:30 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by sophiecentaur (Post 4242915)
One has to chuck out an immense amount of baggage to get there. Most of it came from 'Education', too.

Chuck it, burn it, and bury it!

Emissive Jan25-13 05:30 PM

Re: Why are microwaves reflected by the door?
 
Interesting.

I would have thought we could say a photon approaches the screen? It is one packet of em energy.

Don't we only ever "see" em waves as photons or discrete packets of energy...We don't get two halfs of a photon showing up. It is only when they transfer energy that they become visible everything in-between is a mathematical model. we could emit one photon at a time with a break - thats a series of photons isn't it?

Drakkith Jan25-13 05:40 PM

Re: Why are microwaves reflected by the door?
 
Quote:

Quote by Emissive (Post 4242921)
Don't we only ever "see" em waves as photons or discrete packets of energy...We don't get two halfs of a photon showing up. It is only when they transfer energy that they become visible everything in-between is a mathematical model. we could emit one photon at a time with a break - thats a series of photons isn't it?

It is in the sense that you measure the energy of the EM wave one packet at a time, with a large time between each packet being delivered to your sensor. However you cannot claim that during transit the photon itself was moving like a little bullet or anything. That simply cannot explain things like interference that are properties of waves.

sophiecentaur Jan25-13 05:53 PM

Re: Why are microwaves reflected by the door?
 
If a photon is detected by a sensor in a certain place in front of the screen then all you can say is that it was detected there. You can say nothing about how (what route) it got there or how big it is/was - except by describing the wave behaviour of the energy and, if you want to, saying that relates to the statistical probability of detecting the photon. The photon is only there when it's being detected. You have no idea how it got there as a photon and if you go looking for a photon somewhere else and detect one, it may affect the probablilty of finding one in the original spot.
This is like trying to decide which slit a photon chose to go through one of the 'two slits' in that well known experiment. If you have a detector to 'catch all the photons' that go through one slit, you get no interference pattern because you have eliminated the possibility of a photon going through that slit.

Emissive Jan25-13 06:25 PM

Re: Why are microwaves reflected by the door?
 
Agreed.

But the interference patterns which manifest in locality is maths - a vector space designed to construct and assertian the photons properties but its not actual space in the real world - so I don't see a problem in visualising a photon as point as that is how is shows up (albeit impossible to ever actually locate)......it certainly doesnt move like a bullet as in classical mechanics (because its a wave in it vector space and a bullet is a line in its vector space).


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