# EM waves

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#### Dx

Hiya!

I have some general questions about EM waves since i am somewhat lost from reading and may have misuderstood what I read.

1) does all EM waves travel through a vacuum? if so are they the same speeds/proportional/inporportional to their frequency.

2) I now that EM waves are transverse, correct? but can they travel through pretty much anything?

3) Lets say in a vacuum, the velocity of all EM waves are they nearly 3.00 x 10^8 m/s or what?

Now for my homework question? An EM wave traveling to the east. AT one instant at a given point its E vector points straight up. what is the direction of its vector?

my answer is south unless my right hand rule i am using is actually my left hand facing in some wierd direction. can you help me please or am i correct?

Thanks!
Dx

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A

#### Alexander

Originally posted by Dx
A mathematician is a blind man in a dark room looking for a black cat which isn't there
Really? How come then that a mathematician is the only one who finds the cat (=is always right)?

D

#### Dx

Re: Re: EM waves

Originally posted by Alexander
Really? How come then that a mathematician is the only one who finds the cat (=is always right)?
fair enough but how come you didn't answer my question? Please dont tell me your link is the answer to my question alexander. I wanted some better assistance but your has been appreciated, sir. Thank You!

#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Dx
1) does all EM waves travel through a vacuum?
Come on, Dx, this is an easy one. When you listen to the radio in your car, the EM waves traveled through the air (not vacuum). When someone gets an X-Ray, the EM waves travel through their body (not vacuum).

et cetera...

if so are they the same speeds/proportional/inporportional to their frequency.
Look up "index of refraction" in the chapter on EM waves. The explicit relationship should be there.

2) I now that EM waves are transverse, correct?
Correct.

but can they travel through pretty much anything?

3) Lets say in a vacuum, the velocity of all EM waves are they nearly 3.00 x 10^8 m/s or what?
Yes.

Now for my homework question? An EM wave traveling to the east. AT one instant at a given point its E vector points straight up. what is the direction of its vector?

my answer is south unless my right hand rule i am using is actually my left hand facing in some wierd direction. can you help me please or am i correct?
Yes, you got it right.

#### FZ+

1) does all EM waves travel through a vacuum? if so are they the same speeds/proportional/inporportional to their frequency.
Well, technically they travel in the spaces between atoms in air, which are empty and hence vacuums. They all have the same speed, c in that vacuum. But when they travel through air, they are delayed by hitting stuff and being reemitted, so their overall speed is reduced.

BUT by some variants of DSR, which is a new theory going around, photon energy does affect their speed. But this is only apparent in extreme circumstances.

2) I now that EM waves are transverse, correct? but can they travel through pretty much anything?
Yes, they are transverse. But when they travel through dense matter, they tend to get dissipated/lose their energy when they bump into the particles. Usually, high energy photons - eg. gamma rays get dissipated less, and penetrate further. It is possible to shield from low energy em radiation. Close your eyes, and there is your example.

3) Lets say in a vacuum, the velocity of all EM waves are they nearly 3.00 x 10^8 m/s or what?
According to current theories, yes.

D

#### Dx

Okay!

THANKS!

FZ+ and Tom, take a bow!

Thanks for the clarification. Its much appreciated and clearer now.

Dx

"EM waves"

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