Microwave Oven: Why Are Grid Holes So Small?

[this_tim]

Since the wavelength microwaves used in microwave ovens have a wavelength of 12cm, why are the holes in the grid behind the glass so small? If they were 12cm would the microwaves still stay confined? If the small holes are only to "play it safe" that seems irrelevant since microwave has a "fixed" wavelength. Very confused about all of this and can't seem to find a very clear explanation anywhere.

 

[phinds]

Since the wavelength microwaves used in microwave ovens have a wavelength of 12cm, why are the holes in the grid behind the glass so small? If they were 12cm would the microwaves still stay confined? If the small holes are only to "play it safe" that seems irrelevant since microwave has a "fixed" wavelength. Very confused about all of this and can't seem to find a very clear explanation anywhere.

The idea is to provide a mesh that is effectively a solid wall to microwaves but which can be easily seen through for convenience.

 

[tech99]

Since the wavelength microwaves used in microwave ovens have a wavelength of 12cm, why are the holes in the grid behind the glass so small? If they were 12cm would the microwaves still stay confined? If the small holes are only to "play it safe" that seems irrelevant since microwave has a "fixed" wavelength. Very confused about all of this and can't seem to find a very clear explanation anywhere.

EM waves will not propagate through a hole that is smaller in diameter than half a wavelength. But there are still local fields that will exist on the outside. These are associated with the voltage across the hole and currents in the metal. By making the holes small, these fields are reduced in extent.

 

[this_tim]

EM waves will not propagate through a hole that is smaller in diameter than half a wavelength. But there are still local fields that will exist on the outside. These are associated with the voltage across the hole and currents in the metal. By making the holes small, these fields are reduced in extent.

tech99, could you explain more about those "local fields" such as where they come from and why they would be a problem. I haven't much knowledge of physics, but am a programmer of 20+ years and though the two aren't exactly related, I "may" be able to follow along if you explain a bit more so I can better understand the size of the holes. It really does have me curious - Thanks!

 

[tech99]

tech99, could you explain more about those "local fields" such as where they come from and why they would be a problem. I haven't much knowledge of physics, but am a programmer of 20+ years and though the two aren't exactly related, I "may" be able to follow along if you explain a bit more so I can better understand the size of the holes. It really does have me curious - Thanks!

A hole in a metal sheet is a slot antenna. All antennas radiate by accelerating electrons, but to do this they need to have an accelerating voltage across them. This does not radiate energy itself but it does create an electric field close to the antenna in which electrical energy is stored. There is a similar magnetic field, which arises because, after being accelerated, the electrons have velocity. These fields are sometimes called the Reactive Near Field, and they often store much more energy than the antenna radiates each cycle.
Very small antennas do not radiate very much energy but can have strong Reactive Near Fields.
For the case of a hole or slot in a sheet of metal, field lines can bend through the hole and be observed on the outside, even when radiation is very small. In principle, these fields could supply energy to the body, for instance, to the eye, if placed very close, and thereby constitute a radiation hazard. By making the holes small, both the radiated and Reactive Near Fields are kept very small.

 

[this_tim]

tech99, Oh wow - finally something that makes a whole lot more sense, and a breakaway from the typical thinking in regard to just the waves themselves. Thanks so much for taking the time to provide the explanation. Of course I don't yet understand the complete details of what you wrote, but can picture it in my mind, and have a lot I can look further into as a result of the information you provided.

 

[sophiecentaur]

By making the holes small, both the radiated and Reactive Near Fields are kept very small.

. . . . and, for a (perfectly) conducting screen, the Energy is reflected (conservation has to apply). This is the same as for a continuous sheet.

 

[this_tim]

. . . . and, for a (perfectly) conducting screen, the Energy is reflected (conservation has to apply). This is the same as for a continuous sheet.

Am I correct in guessing that a "perfectly" conducting screen is very difficult, if not impossible to achieve? And please say more about your "conservation has to apply" statement.

 

[sophiecentaur]

Am I correct in guessing that a "perfectly" conducting screen is very difficult, if not impossible to achieve? And please say more about your "conservation has to apply" statement.

Pretty low resistance is ‘good enough’ and Conservation just means the Energy has to go somewhere. It has to be reflected if it’s not absorbed.

 

[this_tim]

Pretty low resistance is ‘good enough’ and Conservation just means the Energy has to go somewhere. It has to be reflected if it’s not absorbed.

Makes sense. Thanks!