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Thread: The speed of light in various mediums, and dielectric constants.

  1. #1 The speed of light in various mediums, and dielectric constants. 
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    A discussion I was having with an Engineer in RF energy I know, resulted in him telling me that the speed of any eletromagnetic waves (including light) has a maximum speed which is well known as the invariant deemed "C"- He continued to say that this speed will be slowed by different mediums depending on their dielectric constants, and that the effects of the dielectric constants will be different depending on the wavelengths of the electromagnetic waves.

    Do any of you have more info about these dielectric influences on EM energy, or what what causes them to have these dielectric constants?
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    Quote Originally Posted by mayflow View Post
    A discussion I was having with an Engineer in RF energy I know, resulted in him telling me that the speed of any eletromagnetic waves (including light) has a maximum speed which is well known as the invariant deemed "C"- He continued to say that this speed will be slowed by different mediums depending on their dielectric constants, and that the effects of the dielectric constants will be different depending on the wavelengths of the electromagnetic waves.

    Do any of you have more info about these dielectric influences on EM energy, or what what causes them to have these dielectric constants?
    Seriously, Mayflow, google is your friend. Start there. Please do some work on your own. Don't expect everyone else to spoon-feed you, especially given your habit of rejecting information that doesn't conform to your tastes.
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    Quote Originally Posted by tk421 View Post
    Seriously, Mayflow, google is your friend. Start there. Please do some work on your own. Don't expect everyone else to spoon-feed you, especially given your habit of rejecting information that doesn't conform to your tastes.

    Well, I have searched online some, but there is a lot of junk to find the jewels in online, and even the jewels are often very incomplete. The prime purpose of a physics forum must be about discussing physics I presume, and if anyone who is actually a live person has knowledge or interest in this, the idea of a forum is just made for that.

    I am interested in this partly because of a project I was charged with at work in order to validate an anechoic chamber. I get almost no engineering support, but I do know it is important to mount a receiving probe with a dipole antenna on some sort of low dielectric material. I talked with the chief engineer and manager of RF materials and he said that styrofoam is typically used but it is difficult to mount the probe accurately enough both in vertical and horizontal geometries, and in the varying distances required at different wavelengths. He recomended to use rohacell. I don't want to get too intricate about it in just one post. If anyone wants to know more or discuss more on the project (with a couple of enterprising co-workers, we were able to ace the project) - but it did bring up some interest in me about the varying effects of differing substances on electromagnetic waves.

    It is obvious to me that certain substances have a lot of varying effects on EM waves, but I hadn't thought about how they actually vary the speed of the waves, and not just the interactions.

    It was actually from a forum - mostly by some things you yourself, TK said that helped me get interested in this. Google is not like discussing with real live people.

    What I did find in searching online so far is that is that the speed of the EM waves (including light of course) is probably not quite as high in air as it would be in a vacuum. It also seems that the speed may vary due to temperatures and frequencies as well.
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    Quote Originally Posted by mayflow View Post
    Well, I have searched online some, but there is a lot of junk to find the jewels in online, and even the jewels are often very incomplete. The prime purpose of a physics forum must be about discussing physics I presume, and if anyone who is actually a live person has knowledge or interest in this, the idea of a forum is just made for that.

    What I did find in searching online so far is that is that the speed of the EM waves (including light of course) is probably not quite as high in air as it would be in a vacuum. It also seems that the speed may vary due to temperatures and frequencies as well.
    What you should have been able to discern is that there is a parameter, called the dielectric constant, that tells you the effect on light speed. There are many tabulated values of the dielectric constant for various materials. It is a function of composition and frequency, among other factors.

    As I mentioned in my post in the other forum, lenses work precisely by varying the speed of light. If you don't want a lense effect, you want a material whose (normalized) dielectric constant is as close to unity as possible. That's the condition you're trying to approximate in the case of EM measurements that attempt to emulate free space (or a vacuum).

    Keeping dimensions small compared to the smallest wavelength of interest will also help minimise the perturbative effects of any material you introduce into the chamber.

    To understand these factors more than qualitatively, you'll need to study Maxwell's equations. Someone else here mentioned the Princeton "In a Nutshell" series. I've never used them myself, but a brief perusal looks promising. You might take a look and see if the volume on Classical Electromagnetics is within your grasp. You'll need to get up to speed on vector calculus (which means you'll have to get up to speed on ordinary calculus first). It's not expertise one acquires casually from a few exchanges on a forum, nor from watching youtubes. You'll have to put in a lot of effort.
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    Quote Originally Posted by mayflow View Post
    A discussion I was having with an Engineer in RF energy I know, resulted in him telling me that the speed of any eletromagnetic waves (including light) has a maximum speed which is well known as the invariant deemed "C"- He continued to say that this speed will be slowed by different mediums depending on their dielectric constants, and that the effects of the dielectric constants will be different depending on the wavelengths of the electromagnetic waves.

    Do any of you have more info about these dielectric influences on EM energy, or what what causes them to have these dielectric constants?
    The speed of light in a material is a function of BOTH its permittivity AND its permeability, . You will need to study , as tk421 mentioned. Start with the wiki articles.
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    Ok, I will just take what I can sort of understand is permeabilty relates to the magnetic fields which are oriented transverse to the electric fields (permittivity) and somehow the square root of some measurement of these multiplied by each other and then divided into 1 it means something or another? Just give me the units in the equation and tell me how they are derived, if you can and will.

    Is it that you are multiplying some units of permittvity times some units of permeability and then dividing them into 1 to get the reciprical?
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    Mayflow: the expression is akin to the mechanics expression for shear wave velocity v = √(G/ρ). The G here is the shear modulus of elasticity, the ρ is the density. See for example this book. Permittivity is something like how easy is it to make space wave whilst permeability is something like how good it is at bouncing back. The reciprocal is there because permittivity is a "how easy" measure.
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    I am not very familiar with permittivity or permeability, but I am pretty sure permeability relates to the magnetic flow of the EM field, and pemittivity relates to the electric potential of the field.
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    Do your own research. Google on permittivity deformability.
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