# Question regarding the simple microwave...

#### mentaaal

Joined Oct 17, 2005
451
Hey there guys, just wandering if any of you could answer a question that has been troubling me for a while...

The metal mesh grill in the front cover of the microwave is in place to stop microwaves passing through and letting microwave radiation entering the environment and possibly harming us right? Well how is it that no microwaves are simply diffracted through the little holes?

Cheers!

#### Dave

Joined Nov 17, 2003
6,970
The wavelength of a typical microwave used in microwave oven is 12.24cm long - think of how small your hole is in your mesh. Additionally, light (which is also an EM wave) has a much smaller wavelength can pass through the mesh quite easily.

Dave

#### Papabravo

Joined Feb 24, 2006
16,787
Hey there guys, just wandering if any of you could answer a question that has been troubling me for a while...

The metal mesh grill in the front cover of the microwave is in place to stop microwaves passing through and letting microwave radiation entering the environment and possibly harming us right? Well how is it that no microwaves are simply diffracted through the little holes?

Cheers!
See the wiki on the subject of diffraction
http://en.wikipedia.org/wiki/Diffraction

#### mentaaal

Joined Oct 17, 2005
451
Hey dave, firstly thanks loads for explaining that for me...

although i am just unsure on one more thing. I did physics in school and have looked up the subject in google but they are not specific on the limit size of the wavelength before diffraction doesnt take place and reflection occurs. Like say for example if that particular microwave was of a smaller wavelength such as that it would diffract through the mesh. Is there a ratio of wavelenth to apperture or a formula or something that specifies what the grating apperture hole size should be?
This is just for curiousity's sake.

#### Dave

Joined Nov 17, 2003
6,970
Hey dave, firstly thanks loads for explaining that for me...

although i am just unsure on one more thing. I did physics in school and have looked up the subject in google but they are not specific on the limit size of the wavelength before diffraction doesnt take place and reflection occurs. Like say for example if that particular microwave was of a smaller wavelength such as that it would diffract through the mesh. Is there a ratio of wavelenth to apperture or a formula or something that specifies what the grating apperture hole size should be?
This is just for curiousity's sake.

I'm not sure about a standard or equation, but by convention they usually say the aperture should be no more than 1/4*wavelength in size. In reality you should be aiming for an aperture no bigger than 10-20 mm. This 1/4 ratio is extensively used in RF and microwave heating as a design convention, from waveguide corners to cavity sizes.

I'll look in Metaxas and Meredith and see if it sheds any light on the origins of these conventions.

Dave

#### mentaaal

Joined Oct 17, 2005
451
cheers dave your a legend! dont worry to much about it i dont want you to go to all the trouble besides it was only out of curiousity that i asked that question. I am wanting to electronic engineering in college this year and wanting to go into telecommunications so i will prob need to know all about that sort of this in time to come but i am happy enough with that answer!

#### Dave

Joined Nov 17, 2003
6,970
cheers dave your a legend! dont worry to much about it i dont want you to go to all the trouble besides it was only out of curiousity that i asked that question. I am wanting to electronic engineering in college this year and wanting to go into telecommunications so i will prob need to know all about that sort of this in time to come but i am happy enough with that answer!
No problems. Actually revise that aperture size down to less than 10mm tops for due to safety considerations.

Good luck with your college application, and feel free to ask further questions if required.

Dave

#### mrmeval

Joined Jun 30, 2006
833
Oddly enough terahertz waves are difficult to use since filtering is difficult. Researchers found that a metal sheet with a specific hole pattern worked just fine.