# Questions Pertaining to Electromagnetic Wave and James C. Maxwell

1. Nov 19, 2013

### dwn

I have a couple questions pertaining to electromagnetic waves and Maxwell's equations. I will list them numerically, for clarity:

1. From the textbook, "once [wave] is created it is self-sustaining and independent of the source." Is this statement only true when it exists within a vacuum? I understand that the permittivity of differing EM waves varies based on the material/environment, so is it safe to say that the intensity of EM waves depends upon distance and medium? The medium does not seem to be a factor because there is no "medium constant" used in any of the EM equations, but it seems like there should be something of the sort.

I don't want to make my question too long, but I want to make sure I get my point across clearly. For example, the sun emits radiation directed towards the sun, of which only a small percentage (28%) actually journeys to the surface of the earth. Of that percentage, does any of that radiation pass unscathed through the entire body of the earth and continue on into space? Or is all that energy that is not reflected back into space, absorbed and transformed into heat on Earth?

2. E and B are not spatial vectors when looking at the graphical image of an EM wave. What does it represent--the motion of the particles? What is the reaction of the space around the EM wave? Is this similar to a string wave, in which the string itself is not moving, but the intensity of the wave is observed by the peaks and troughs of the wave at that point...?

(3) How in the universe did these incredibly brilliant masterminds discover all of this!? It blows me away and seems like something from science fiction! They practically had sticks and stones in comparison to what we have today and were able to understand and create something so intangible------I feel stupid. end;

2. Nov 19, 2013

### ZapperZ

Staff Emeritus
You must identify the textbook. It is time you learn to do that in here whenever you are referring to ANY source. Cite name of author, and title of the book, at the very least. If you are citing specific passages, indicate the page number and possibly the edition number if the text has multiple editions.

Zz.

3. Nov 19, 2013

### gabriel.dac

What?

Anyway, I know what you mean. As far as I know, Earth either absorbs or reflects the radiation. If it can cross the entire planet, then that is news to me

4. Nov 19, 2013

### dwn

My apologies Zz, I didn't think that would be of any importance. I will make sure to do so in future posts.

Title: Physics: A Strategic Approach (2nd Edition)
Author: Randall D. Knight
ISBN: 978-0-8053-2736-6

5. Nov 19, 2013

### dwn

yes, the Earth.. I'm asking whether or not there is any form of radiation that can permeate any medium.

@ Zz: The page number was 1105 under the section titled Antennas.

6. Nov 19, 2013

### dwn

Another questions: If a radio wave is emitted by some radio station in Punta Arenas, S. America, how do they (1) limit unwarranted interception (i.e. military operations) and (2) broadcast to multiple stations all over the world from just one source?

7. Nov 19, 2013

### sigma_

Look at the wave equation for an EM wave through a medium. It includes the coefficient n2/c2, where n is the index of refraction of the medium. It does indeed take into account the medium.

To answer the second paragraph of your question, perhaps this rather simplistic point might be of assistance.

When you shine a light over a solid object of moderate density/thickness (say a sheet of ply wood), does it not cast a shadow, or does the light go straight through it? So at the very least, visible light radiation is either absorbed or reflected backward almost entirely. Higher energy radiation has greater penetrating power with respect to solid objects than visible light does, but I am not aware of any sort of EM radiation being able to penetrate something as dense and voluminous as the Earth. Perhaps I am understanding your question incorrectly.

8. Nov 19, 2013

### dwn

No, Sigma, you understood correctly. I know that the majority of EM waves are either reflected or absorbed; however, I thought when you got into gamma rays, or even when you get into quantum physics (which I clearly know nothing about) that these things pass freely through all mediums. I was just curious as to how far the physics went...thank you for answering my question though.

9. Nov 19, 2013

### Khashishi

EM waves are attenuated as they pass through media. The energy is absorbed by the media.

You must be talking about the Earth's albedo. The Earth reflects most of the light, and absorbs the rest (heating the Earth). None of the sunlight passes through the Earth. (In principle, some negligible fraction light can pass through, but far too little to be of any physical consequence.) The Earth cools off by re-emitting the energy mostly as infrared.

The E and B vectors represent components of the antisymmetric electromagnetic tensor. The electromagnetic tensor just exists as part of the model. It can't be reduced into some motion of particles or represented as some simpler thing. You just have to accept it.

Lots of hard work.

10. Nov 19, 2013

### sophiecentaur

A cannot think of any frequency of em waves that will pass through the Earth without being absorbed. There are particles, neutrinos, which will do it, though. Water is 'transparent' to visible light but only when you are dealing with distances of up to 100m or so. The sea bed, at depth, is in darkness so we are only talking in relative terms. Very long wavelength radio waves will penetrate quite a long way beneath the surface (skin effect) but not right through. An em wave with a wavelength comparable with the Earth's diameter ( less than a Hz in frequency) would diffract round it - but that's not 'through' it.

BTW, look up Ground Wave propagation of radio waves. That should throw up some interesting links for you.

11. Nov 19, 2013

### Khashishi

Gamma rays are fairly penetrating, but the Earth is really, really big. The longer the distance, the higher the chance of absorption. So gamma rays won't penetrate much more than a little bit of the surface of the Earth.

12. Nov 19, 2013

### sigma_

I'm not sure about (1), but I believe for (2), they bounce their signal off of the ionosphere.

13. Nov 19, 2013

### Jano L.

The quantities $\mathbf E, \mathbf B$ are vectors which at any given point give force that would act on a test particle, if it was placed there and the field due to other sources were the same (the charge $q$ of the test particle would have to be small if we really wanted to perform the test). The force on a particle at $\mathbf r$ is given by the Lorentz formula, which in the SI system is

$$\mathbf F= q\mathbf E(\mathbf r) + q\mathbf v\times \mathbf B(\mathbf r).$$

That is the meaning of the fields.

14. Nov 20, 2013

### sophiecentaur

Is that really necessary in a case like this? Go to pretty much any text good and it will tell you the same in different words. Strictly referencing a source on a forum like this should only be necessary when there is something controversial or new.
Are you suggesting that the quoted passage is not right? (Would anyone?)

15. Nov 20, 2013

### dwn

I've heard neutrinos mentioned before, possibly on "Through the Wormhole" or some other science show and I must be tangling these two very different entities--being that neutrinos carry no charge.

Thank you for letting me know about ground wave propagation, really fascinating stuff. I read several articles on the topic and it really helped to clarify and drive home this point.

16. Nov 20, 2013

### dwn

I'd like to start of by saying, I know why the sky is blue.

My next question would be: how does space absorb light? I understand that there are practically no molecules to reflect/absorb the light in empty space (The Great Hoover), but what happens to the light at the 'edge' of the universe? Is this what causes the expansion of the universe? Energy is neither created nor destroyed, so what happens to it, if it does not come into contact with any object in space?

I know these questions are probably very elementary, but I have to start somewhere! Thank you!

Last edited: Nov 20, 2013
17. Nov 22, 2013

### Khashishi

Empty space does not absorb light. But small amounts of matter will cause some absorption and scattering. There is no edge of the universe. Either it extends forever, or it wraps around in some way. Nobody has made a credible case for an edge.

18. Nov 22, 2013

### Jano L.

But that could be said about the above two possibilities as well. Universe as a box with edges or periodic boundary conditions (which is similar thing) isn't that much worse. It's the simplest way to model finite universe - great as a boundary condition for partial differential equation:-)

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