Why Microwave Ovens Can Have See Through Windows

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microwave ovens, clear glass doors

Have you ever wondered why a microwave oven can have a see-through window? “Microwaves” are radiation after all. We all know about tempered glass as commonly used in convention ovens, but microwave ovens also have a perforated metal mesh screen on the inside of the glass. How does this help?

How is this perforated screen on microwave ovens effective against microwave radiation?

Beginning with the basics; microwaves are in fact, radiation. High frequency radio waves to be exact, but radiation all the same. But so is any form of heat, light, sound, etc. They too are radiation. Throwing a baseball is in fact, a form of ‘radiation’ in that the ball is ‘radiating away from’ where it started. In short, any linear path movement is radiation and the trajectories of a baseball or a golf ball etc. are in the strictest sense, radiation.

In electromagnetism, there are two concerns; mutagenic (ionizing) and non-mutagenic (non-ionizing) radiation. The non-ionizing type are what microwaves are. In fact, on the EM scale microwaves fall below ordinary heat-lamps in their radiation output. Microwave radiation is a form of high frequency radio wave but still of the non-ionizing type. No charged ions are produced when the microwave passes through matter, but it has just enough energy to produce molecular excitation which is turn creates heat. This is how a microwave ‘cooks’ food. Even if a microwave oven had a bad seal or other ‘hole’ in it, during normal operation it would not be lethal. Placing your face or fingers near the opening during use would give you a burn rather quickly however. People with pacemakers should stay well back from a microwave oven in use as microwave radiation is after all, radio waves which could affect the ‘timing’ and thus, the proper functioning of the heart implant device.

Modern Microwave Oven

microwave oven

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The Faraday Cage

In 1836, physicist Micheal Faraday built a container whose walls were of a conducting material that was intended to absorb and redistribute any external electrical field, so as to cancel the field’s effect upon the interior of (and any contents therein) of the container. See the image below. If you think “black box” like airplanes carry, you have the right idea. The ‘black box’ carried on airplanes are apart from being fire and crash-resistant are a useful example of a ‘Faraday Cage.’

 Faraday sheild

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Faraday Cage and Some Uses

It is probably based in some fact that some High Schools and even proctored exams in college might use a simple Faraday Shield in the testing hall to prevent students from cheating by texting for answers from their cell phones, hand-held or blackberry devices. The classroom might have the windows covered in aluminum foil or some other similarly conductive ‘shielding’ material to prevent radio waves from being able to penetrate into the room thus, blocking cell phone reception.

Computers must by law not interfere with TV or radio reception (and must accept any interference that they receive) so again, a Faraday shield is used. The electronic components that emit potentially disruptive EM radiation are containerized behind either solid metal shielding or a suitable mesh ‘envelope’ whose holes are ‘substantially smaller’ than the frequency of radio wave that they emit. Shielded coaxial cable is also a Faraday shield, for incoming/outgoing signals are effectively blocked and do not cause disruption to other external devices otherwise capable of receiving the proximal leaked signals. A damaged or ‘partially skinned’ coaxial cable will ‘leak signal’ resulting in the device receiving poor quality reception or display, and other non-related devices would receive leaked static or other disruption. Television audio coming over portable and civilian band (“CB”) radios (and vice-versa) in the immediate vicinity comes to mind.

To protect my magnetic media (floppy disks, tapes, ZIP-drives, etc.) from EM radiation, I use an Army Surplus Ammo can, whose thick metal sides act as a shield to radio waves and EM radiation. This offers at least some protection from the computer monitor, power cables and cords, TV and the refrigerator. There is probably some protection from weak magnets too but I would not wish to place refrigerator magnets on the ammo box to test the facts because thickness and even multiple layers are required for true, 100% protection from all Electromagnetic radiation sources.

This also explains why in an automobile accident involving electrical power lines draped over it, it is safer to remain inside the vehicle than trying to exist. There exists an apocryphally bordering upon urban legend that it is the rubber tires under the car protecting you. Not so true. It is really the fact that you are surrounded by a ‘Faraday cage’ of metal, which disperses the electrical strike away and shields the interior. And again, the same with buildings of concrete with iron rebar and plaster walls, which is also a Faraday shield to some extent. This is why cell phones tend to fail inside of larger buildings. Higher frequency radio waves with ‘tighter’ or ’smaller’ sine wave troughs can penetrate deeper through ‘holes’ in the building (windows, cracks around doors, elevator shafts, ventilation shafts, etc.) and thus, are becoming less prone to suppression and dropped signal.

I want to try an experiment sometime to test my ammo box’s ability to stop cell phone frequency. What I would do is while playing music media here in this room, use my cell phone to call my ‘standard’ phone in the other room. Make the call and leave the cell phone here while I go answer the standard phone in the other room. I should hear music playing.

Then, return to the cell phone and place it, -still live, into the ammo box and close the door and lock it. The cell phone is now enclosed in a Faraday Cage/Shield. The ammo box is not insulated so sound waves will be audible inside the metal box, but the cell phone’s radiation (the broadcast frequency) will be contained. Checking the ‘standard phone’ should now indicate silence, no music would be heard, as the cell phone is no longer able to broadcast its signal out even though the sound would still be heard inside the ammo box. To test that, a simple mini cassette recorder placed with the cell phone would show that yes, the ammo box is not sound-proof. The sound would be muffled no doubt, but still audible.

Microwave Oven Mesh on Door

 microwave oven with food splatters on the glass

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So, this is the reason for the holes on the metal screen of a microwave. These holes are substantially smaller than the penetrative ability of the microwave’s sine waves to pass through, effectively keeping them locked within the confines of the oven. Light can pass through the perforated metal screen and glass window, but microwave radiation at the frequency of the oven is effectively blocked by the Faraday Shield effect!


Posted on Sep 19, 2009
Jerry Walch
Posted on Sep 19, 2009