Light and Faraday Cage confusion

[mbaron13]

Hi guys,

The other day I found myself asking the question below and couldn't quite manage to re-align my own personal experience with E&M theory. I am sure it will just be a gap in my knowledge base of E&M.

The Question:

If you are contained within a metal mesh cage which acts as a Faraday cage why can you still see out beyond the cage?

Surely if a Faraday cage shields the interior from external electromagnetic radiation you should not be able to see out beyond the cage as no 'external' light enters the cage and therefore your eyes.

Any help would be much appreciated.

 

[BruceW]

hey, welcome to physicsforums :) Well, the simple answer is that the metal mesh cage is not a perfect Faraday cage. a 'true' Faraday cage will block any and all EM fields. But because the metal mesh cage has holes in it, it is possible for EM radiation to travel through the holes. The metal mesh Faraday cage should still block the EM fields which have a wavelength much greater than the holes, because the charge should be able to move around sufficiently enough to block the EM radiation. So, essentially, the higher-frequency EM radiation is let in because the charge is restricted to the metal mesh, and it can't travel around wherever it wants to be able to stop the higher-frequency EM radiation.

 

[voko]

Well, the simple answer is that the metal mesh cage is not a perfect Faraday cage. a 'true' Faraday cage will block any and all EM fields.

Well, it is not truly "Faraday" because the shielding effect of metallic enclosures was first described by Franklin.

Ignoring that, I disagree with your terminology. As far as I know, Faraday made his cage specifically to demonstrate that no static electric field or charge could enter the cage. He was not concerned with EM waves; in fact, it took about thirty years after his experiments with his eponymous cage for the concept of EM waves cage to be predicted theoretically, and another twenty years for them to be confirmed experimentally.

So a metal mesh cage is a perfectly true Faraday cage because it does what it was originally conceived for. That it can also shield from EM is a bonus. BTW, it should also be remarked that even a fully solid sheet of metal may still let some EM radiation through. I am not talking about X and gamma rays. Any EM field penetrates into conductors, so this is a question of thickness and intensity.

 

[mbaron13]

Thanks for the welcome and the comments.

Voko:

1. If my terminology is wrong (for historical reasons), what is right? E&M shielding effect? I was under the impression that, though you are right about Faraday demonstrated only for static fields (I was already aware of this), the name Faraday cage has just stuck and encompasses shielding of all E&M fields these days. I would love to here your take on this?

2. Are you referring to the Skin Effect when you are referring to penetration of fields into conductors? Am I right in thinking that for high frequency waves (such as light waves) the skin depth is so small that it is practically impossible to create light waves so intense that they would penetrate even the thinnest of metal sheets, this is why we cannot see through a full solid sheet of metal.

 

[voko]

1. I think "shielding" is the correct term. I have seen "Faraday cage" used where "shielding" would be more appropriate. Especially because Faraday's intent was to prevent external field/charge from getting in. These days, people sometimes talk of placing equipment inside a Faraday cage with the intent of preventing sensitive emissions from leaking out. Perhaps the term "cage" conveys a feeling of something more secure than is afforded by mere "shielding" :)

2. Skin effect is different but related. Anyway, "depth" is a bit subjective. There is no hard boundary. Rather, the field is attenuated steadily as it penetrates inside the conductor. It is not correct that even thinnest of metal sheets are opaque. Very thin foil is (semi)transparent. Note that much depends on the frequency of the incident radiation. Above the so called plasma frequency (UV and up for most metals) metals become transparent. Which is a problem for optics in those bands: you cannot make a mirror!

 

[UltrafastPED]

If the holes are smaller than the wavelength of the EM radiation then you have an effective Faraday cage.

 

[sophiecentaur]

If the holes are smaller than the wavelength of the EM radiation then you have an effective Faraday cage.

And you wouldn't be able to see through it.
Here is another example of sloppy use of terms coming back to bite us. Before 'buying' a 'Faraday cage', it would be as well to look at the manufacturer's spec. They are difficult to make and cost a packet if you want a good one. Getting the measuring signals in and out is a massive problem.

 

[mbaron13]

1. I think "shielding" is the correct term. I have seen "Faraday cage" used where "shielding" would be more appropriate. Especially because Faraday's intent was to prevent external field/charge from getting in. These days, people sometimes talk of placing equipment inside a Faraday cage with the intent of preventing sensitive emissions from leaking out. Perhaps the term "cage" conveys a feeling of something more secure than is afforded by mere "shielding" :)

2. Skin effect is different but related. Anyway, "depth" is a bit subjective. There is no hard boundary. Rather, the field is attenuated steadily as it penetrates inside the conductor. It is not correct that even thinnest of metal sheets are opaque. Very thin foil is (semi)transparent. Note that much depends on the frequency of the incident radiation. Above the so called plasma frequency (UV and up for most metals) metals become transparent. Which is a problem for optics in those bands: you cannot make a mirror!

Thank you Voko, very interesting - I have never heard of the plasma frequency...I shall do some reading up on this. Also, I have never really thought about it but the use of the word "depth" is a bit misleading - you are right in saying there is no hard boundary.

 

[sophiecentaur]

The transparency of metals to high frequencies is a problem for Xray telescopes but they get over it by using an oblique angle on the steep sided portion of a paraboloid and avoiding near-normal incidence at the centre. Smart - hey?

 

[mbaron13]

And you wouldn't be able to see through it.
Here is another example of sloppy use of terms coming back to bite us. Before 'buying' a 'Faraday cage', it would be as well to look at the manufacturer's spec. They are difficult to make and cost a packet if you want a good one. Getting the measuring signals in and out is a massive problem.

Sophiecentaur, what do you mean by "measuring signals"? Did you mean measuring the fields inside and the outside of the cage to check the cage is actually functioning as it should is a massive problem ?

 

[sophiecentaur]

I mean getting signals in an out of the measuring equipment (telemetry) (plus the power supplies). A perfect screen would mean that you'd have to use batteries and stay inside or record everything. Stuff creeps in at -60dB to every seam and door 'seal' unless you go to great lengths. The connecting cables have to go through loads of filtering and screening. V costly if you want the best.

 

[mbaron13]

I mean getting signals in an out of the measuring equipment (telemetry) (plus the power supplies). A perfect screen would mean that you'd have to use batteries and stay inside or record everything. Stuff creeps in at -60dB to every seam and door 'seal' unless you go to great lengths. The connecting cables have to go through loads of filtering and screening. V costly if you want the best.

I don't have any practical based experience in this and so must confess have never thought about it from a purely "trying to get the measurement" perspective. Performing measurements correctly must be a nightmare.

 

[anandr]

I don't have any practical based experience in this and so must confess have never thought about it from a purely "trying to get the measurement" perspective. Performing measurements correctly must be a nightmare.

Sometimes you may simplify your measurements by using clever (or "very similar") system depending on the wavelength range of interest and precision you want to achieve.

For example, here: http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=5510 you may find commercially available wire grid polarizers for visible and IR radiation. Although they are not exactly the "Faraday cage" or "EM shield", but principle is very close. These polarizers are made of thin metal wires deposited onto durable transparent surface. Energy of the EM radiation (visible or IR) that falls within the working wavelength range and is polarized along the wires is spent to move the electrons in the wires so very small amount of such incident light comes out of this polarizer.
But if the light is polarized across the wires it does not interact that much with the electrons and relatively small amount of it is wasted (absorbed, reflected, etc.).

If you combine two such polarizers at 90° to each other they allow very little of the incident radiation to pass through. From one side they are polarizers and two crossed polarizers transmit almost no light. But from other side such configuration is essentially the fine wire mesh or "Faraday cage"/"EM shield" for optical wavelengths.

These things are basically optical elements few millimeters thick and one inch in diameter. To measure light transmission through them you need stable light source (like laser or collimated LED) and good reliable detector.