Is it possible for microwave beam to pass through non-metalic materials?

Question

Greetings fellow physicists. I have some questions about the ability of different electromagnetic waves to pass through materials that I hope you can clarify.

It seems that microwaves can go through concrete, wood, etc. since we can listen to the radio inside houses. However, it doesn't make sense to put a TV antenna indoors because the pictures might flake; so it is apparent that radio frequency doesn't have trouble passing through walls and so on, but the TV frequencies have. I'm guessing as the frequency increases, it gets harder for waves to get through non-conductive stuff. That would explain why we can't see inside boxes - visible light reflects instead of going through.

Things get more confusing with X-rays and gamma rays. They seem to move through matter easily, like how X-rays image our skeletons and are used for security screening. Also, radiotherapy uses gamma rays to reach tumours inside patients.

So I'm not sure if microwaves really penetrate concrete and other materials or not. But visible light, x-rays and gamma rays acting so differently is puzzling. Can anyone explain the physics behind why different waves can or can't pass through various materials? Some insight would be greatly appreciated!

Answer 1

For a partial answer to the easy part...

Yes. The "microwave safe" label on many dishes indicates in part that the dish is mostly transparent to microwave radiation of the frequency used in ovens (even if it is opaque to visible), and will heat up much more slowly than something that is mostly opaque to microwave, like water (which is transparent to visible).

Different wavelengths pass differently through different substances. This is a familiar fact from everyday experience of visible light passing through colored glass. Invisible colors can be likewise filtered with the appropriate materials. You are likely familiar also with UV protective glass (more opaque to UV) and greenhouse gases (more opaque to infrared).

You may be interested in looking up penetration depth tables for various wavelengths in various materials.

Answer 2

A little more info:

TV antennas are up on the roof because 1) best reception occurs when a "line-of-sight" exists between the transmitting antenna and the receiving antenna at VHF/UHF frequencies and 2) metal objects like plumbing, conduit, and rebar ("clutter") in your house reflect and deflect VHF/UHF signals, weakening them and causing cancellation via a mechanism called multipath.

Microwaves readily pass through plastics. In fact, fiberglass + resin is used as a weather cover for microwave antennas so the feedhorn assembly won't get iced up in the winter or loaded with bird droppings in summer.

Radio inside houses does not consist of microwaves; AM radio is of such low frequency that the penetration depth of those signals exceeds the thickness of a house, as long as it is not made out of sheet metal.

FM reception also penetrates houses but because it is subject to multipath, a simple indoor antenna that can be oriented for best results is almost a necessity.

Part of the reason that higher frequencies are more problematic/touchy is that when the wavelength of the signal is of order ~size of metal objects in the neighborhood, then all sorts of bouncing/scattering/reflecting become possible. This means longwave transmission (like the AM radio band) is relatively immune to this whereas FM (and higher frequencies) are strongly affected.

Every sort of material (metal, wood, plastic, stone, glass, ceramic, concrete, sea water, you name it) possesses a certain amount of electrical conductivity and a certain amount of dielectric behavior; taken in combination, this means that a beam of electromagnetic radiation may be reflected, refracted, absorbed, or transmitted without loss by the materials it impinges on- and all of that is frequency dependent. Entire college courses are taught on this topic, so this forum is not the right one for going into this in detail.

The picture is different when the beam is of such short wavelength that interactions between the beam and the material it strikes take place on scale lengths of order ~way smaller than the physical dimensions of the object. So X-rays and gamma rays are extraordinarily difficult to reflect or refract under ordinary circumstances. They tend to zoom straight through wood and plastic, and require metal or thick concrete walls to stop them.

Lastly... in the special case where the energy per photon of incoming radiation is a close match to the energy level of an outer electron belonging to an atom being struck by that beam, then all sorts of cool physics comes into play and it becomes possible for certain photons to be strongly absorbed while others are unaffected. In this way you can make glass that absorbs green light and passes red light, or passes IR while blocking UV, and so on.