What Causes the Deflection of Electromagnetic Waves?

[mariush]

Hey guys! This is a question that has been troubling me for a bit.
What causes electro magnetic waves to be deflected? When i close the door to my bedroom, it turns dark. At the same time my radio has a perfect signal, and so does my x-ray reciever.

How come waves in specific wavelength intervals seem to be deflectet when waves with both higher and lower wavelengths pass right through?

Thanks for any insights!
Marius

yep, should've said 'stops', not stoppes..

 

[Forestman]

I can't really answer your question. But this site might help you.

http://science.jrank.org/pages/2368/Electromagnetic-Spectrum.html

 

[Drakkith]

That is a complicated question. Or at least the answer is. Here is some basic info as well.
http://en.wikipedia.org/wiki/Transparency_and_translucency#Introduction

 

[EricTheWizard]

EM waves are deflected based on a variety of parameters, including wavelength, index of refraction, angle of incidence, and conductivity of a solid. General rules of thumb: the more conductive an object is, the more reflective it is. Also, EM waves tend to "ignore" the presence of objects smaller than its wavelength, which explains why radio signals can pass through walls (because their wavelengths are on the order of meters). X-rays, which have very tiny wavelengths (trillionths of a meter) have more energy than radio waves, and while they cannot ignore the presence of most objects like radio waves, they have a lot more penetrating power due to their higher momentum. p=h/2πλ

This topic is closely related to "skin depth", or how deep a wave can penetrate into a surface.
This article might be of help: http://en.wikipedia.org/wiki/Electromagnetic_shielding

 

[Pythagorean]

Also, EM waves tend to "ignore" the presence of objects smaller than its wavelength

EM waves will also ignore objects bigger than their wavelength too. The general rule of thumb is that the wave and the object have to be on the same order for an interaction.

 

[yungman]

Does the Faraday's cage stop EM wave?

I know for a fact at audio frequency, Faradays cage don't block EM at all. Guitar pickups that consist of big coil that pickup magnetic signal can pick up EM 60Hz very well.

 

[chrisbaird]

A perfect conductor reflects all frequencies of electromagnetic waves because the free electrons in the material are able to keep up with the wave and cancel it out internally. Real conductors let the waves penetrate a bit into the material because the electrons cannot perfectly respond to the incident fields. The measure of how much the fields penetrate is the "skin depth". If the object is much thicker than the skin depth at a certain frequency, then no waves get through to the other side. If the object is much thinner than the skin depth, then the wave can get through. The skin depth generally gets smaller for higher frequencies, so that lower frequencies can get through thicker objects.

A non-conductor will have a dielectric response, which means the bound electrons move in response to the EM wave and interact with it. The bound electron response is frequency dependent. Below a resonant frequency of the material, the waves pass through. Near a resonant frequency, the material's electrons respond strongly, and there can be strong refraction, absorption, and reflection of the wave. Above the resonant frequency, things quiet down again and the wave can go through. Real materials have many resonant frequencies whose effects all add up, so it gets complicated pretty quickly

At high enough frequencies, whether a conductor or not, the electrons cannot keep up with the wave and so there is very little interaction. The waves pass on through as if there was nothing there.

If the object is not a solid slab, but has holes, then you have the additional possibility of the waves going through the holes. The holes will act as waveguides. But waveguides only allow through waves about the cutoff frequency. Loosely speaking, long-wavelength waves cannot fit through small holes, so lower frequencies are reflected by an object with holes while higher frequencies are not.

The ability of a Faraday cage to reflect waves of a certain frequency depends on the thickness of its walls, and the size of its holes compared to the wavelength.

 

[Ben Niehoff]

A good example of how a Faraday cage interacts with EM radiation is the window to a microwave oven. These windows have a bunch of little holes so that you can see your food inside. Therefore, visible light passes through the holes, no problem. But the microwaves do not pass through the holes; otherwise they would fry your face off and give you cancer.

The reason this works is that microwaves have wavelengths on the order of a few centimeters. The holes in a microwave oven window screen are on the order of a few millimeters. Since the holes are much smaller than the wavelength of the microwaves, the microwaves do not pass through.

Visible light, however, has wavelengths on the order of a few hundred nanometers, so they pass through the holes easily.

 

[yungman]

A perfect conductor reflects all frequencies of electromagnetic waves because the free electrons in the material are able to keep up with the wave and cancel it out internally. Real conductors let the waves penetrate a bit into the material because the electrons cannot perfectly respond to the incident fields. The measure of how much the fields penetrate is the "skin depth". If the object is much thicker than the skin depth at a certain frequency, then no waves get through to the other side. If the object is much thinner than the skin depth, then the wave can get through. The skin depth generally gets smaller for higher frequencies, so that lower frequencies can get through thicker objects.

A non-conductor will have a dielectric response, which means the bound electrons move in response to the EM wave and interact with it. The bound electron response is frequency dependent. Below a resonant frequency of the material, the waves pass through. Near a resonant frequency, the material's electrons respond strongly, and there can be strong refraction, absorption, and reflection of the wave. Above the resonant frequency, things quiet down again and the wave can go through. Real materials have many resonant frequencies whose effects all add up, so it gets complicated pretty quickly

At high enough frequencies, whether a conductor or not, the electrons cannot keep up with the wave and so there is very little interaction. The waves pass on through as if there was nothing there.
Say if it is copper, what frequency is high enough that it will get through? And at what thickness? can you give an example?

If the object is not a solid slab, but has holes, then you have the additional possibility of the waves going through the holes. The holes will act as waveguides. But waveguides only allow through waves about the cutoff frequency. Loosely speaking, long-wavelength waves cannot fit through small holes, so lower frequencies are reflected by an object with holes while higher frequencies are not.
What is the formula for frequency vs the radius of the hole?


The ability of a Faraday cage to reflect waves of a certain frequency depends on the thickness of its walls, and the size of its holes compared to the wavelength.

Thanks for you time.
Alan

 

[chrisbaird]

Gamma rays will go through copper of just about any practical thickness. Gamma rays are produced in nuclear decay. Even visible light will partially go through copper if it is thin enough. Think of sunglasses coated with a thin metallic layer. They block some of the sunlight, but not all. That's why you can see through them.

For square waveguides, in the lowest mode possible, the cutoff frequency is ω = cπ/a where c is the speed of light and a is the width of the square hole. Below this frequency, light cannot penetrate the hole. See my http://faculty.uml.edu/cbaird/95.658%282011%29/Lecture5.pdf" for more info.

 

[yungman]

Gamma rays will go through copper of just about any practical thickness. Gamma rays are produced in nuclear decay. Even visible light will partially go through copper if it is thin enough. Think of sunglasses coated with a thin metallic layer. They block some of the sunlight, but not all. That's why you can see through them.

For square waveguides, in the lowest mode possible, the cutoff frequency is ω = cπ/a where c is the speed of light and a is the width of the square hole. Below this frequency, light cannot penetrate the hole. See my http://faculty.uml.edu/cbaird/95.658%282011%29/Lecture5.pdf" for more info.

Thanks for your time. So for radio frequencies even like mm wave, conductor shielding is going to be very effective in shielding EM wave. They are nowhere close to even light wave.

 

[LostConjugate]

How come waves in specific wavelength intervals seem to be deflectet when waves with both higher and lower wavelengths pass right through?

Think of the object as a collection of electric charges that fight against the wave as it attempts to oscillate. The higher the frequency the more often the electric charges interfere. The more the wave fights the more it is repulsed. Like quicksand.

 

[ZealScience]

That depends on frequency of the radiation and the configuration of materials that are blocking the wave. For example, normal light can pass through glass but infra red cannot. Or for green glass, red light is absorbed but green light passes. To sum it up, the wall of the room determines what type of wave can pass.

 

[RedX]

Apologies for digging up this old thread, but I have some questions on some of the responses:

A good example of how a Faraday cage interacts with EM radiation is the window to a microwave oven. These windows have a bunch of little holes so that you can see your food inside. Therefore, visible light passes through the holes, no problem. But the microwaves do not pass through the holes; otherwise they would fry your face off and give you cancer.

The reason this works is that microwaves have wavelengths on the order of a few centimeters. The holes in a microwave oven window screen are on the order of a few millimeters. Since the holes are much smaller than the wavelength of the microwaves, the microwaves do not pass through.

Visible light, however, has wavelengths on the order of a few hundred nanometers, so they pass through the holes easily.

But why doesn't the metallic screen inside the microwave window get burnt, as does aluminum foil when you put it in the microwave?

If the object is not a solid slab, but has holes, then you have the additional possibility of the waves going through the holes. The holes will act as waveguides. But waveguides only allow through waves about the cutoff frequency. Loosely speaking, long-wavelength waves cannot fit through small holes, so lower frequencies are reflected by an object with holes while higher frequencies are not.

What you say makes a lot of sense. However, what about the limit of infinite wavelength of light, i.e., static electricity? Surely you can't have huge holes and claim that the object is protected from static fields by the cage?

 

[jpotter0]

"But why doesn't the metallic screen inside the microwave window get burnt, as does aluminum foil when you put it in the microwave?"

The metallic screen is able to transfer away the heat generated by the rf currents flowing on it by conduction and convection.

 

[SteamKing]

About your x-ray receiver, do you live where you get a lot of x-rays coming your way?