Exam Question Today

[circumpolar (Matt)]

Surveying Question regarding EDM (Electromagnetic Distance Measure)

Q1) Is the speed of light and radio waves the same in a vacuum?
YES [my answer]
Q2) Is the speed of light and radio waves the same in the atmosphere?
YES [my answer]

With regards to Q2, I think that the SPEED is the same (C), but the velocity (which also involves a direction) would vary between the two just like different colours (Energy levels) of light refract differently when passing through the atmosphere. Their speed is the same as each other but the distance they traverse in a given space varies due to the refractive index of that medium, and the atmosphere is a vast mixture. I think of a crystal prism spectragraph where you can see that the colours take different pathways.

Your thoughts would be appreciated. I need resolve so I can get some sleep
PS. If you know a good reference please include it.

[Screwdriverone (Chris)]

I would say you are correct here Matt,

Radio and light waves are both Electromagnetic, so barring any disturbance from the atmosphere then they would be the same, both should travel at the same speed and even if they are affected, would probably be affected in the same way and therefore be at the same speed.

Here is a good reference on the subject with a line that probably answers the "typical" wrong answer to this question as people think of sound waves as radio waves as they associate radio with sound.

http://www.aerospaceweb.org/question...my/q0254.shtml

Now, go and get some sleep (if I am correct like you that is)

Cheers

Chris

edit: Funny thing further down the page where someone has listed the distance in miles and km to stars etc and even when they are talking in "quadrillions" of miles etc, its funny to see the disclaimer at the bottom of the table saying "Your mileage may vary"

[circumpolar (Matt)]

Originally Posted by Screwdriverone

"Your mileage may vary"

Thanks Chris.
I think that the question was worded hoping to catch someone out because the EDM on the Theodolite will give the wrong distance measures when used in humid, foggy air. A correction value needs to be inputed into the device and updated as the conditions change throughout the day.
Trick Question me thinks .

[Zuts]

Hi,

The answer to the second question is NO.

The speed of light in a medium like air or glass is governed by its refractive index see http://en.wikipedia.org/wiki/Speed_of_light. For example the speed of light in glass is less than the speed of light in air which is less than the speed of light in a vacume.

I dont believe this applies to radio waves in the same way so i would say radio waves in the atmosphere are faster than light waves.

Regardless, this still means that the speed of a particlar light beam is the same for all observers.

Cheers
Paul

[Screwdriverone (Chris)]

Hmmmmmm, interesting point Zuts, however, further down the page it says that;

"Since the speed of light in a material depends on the refractive index, and the refractive index may depend on the frequency of the light, light at different frequencies can travel at different speeds through the same material. This effect is called dispersion"

so while this may be true for light of different frequencies there is not enough information in the question for a definitive answer to be made in this case. It also doesnt specify what type of "radio" wave is being compared and since light and radio are both waves within the electromagnetic spectrum, yes they will be affected by different media to different degrees, but based on this question, I dont think there is enough information to base the assumption that radio waves would therefore be unaffected or if so, by how much, so it is possible that they are BOTH affected by the atmosphere to the point of becoming the SAME speed again, unlikely, but possible?

Certainly, other waves such as sound actually speed up in media such as water, and if sound waves are accelerated, dispersed or scattered by water or reflected by atmospheric conditions, couldnt the same argument be used to say that the radio waves would also be dispersed or impeded by the medium through which they are passing too? Radio waves are blocked and reflected by other objects such as trees, buildings etc, so why is water or mist in the atmosphere any different?

I know it descends into dark pits of theory and such after this, but as I said, i dont think there is enough information in the question for a definitive NO in this case. (eg. what frequency light, radio waves, what atmosphere? at sea level or ionosphere? et al)

However I may be wrong based on the question being a specific reference to something Matt is studying and therefore its worded this way to make them think - "aah yes, they DONT travel at the same speed in the atmosphere, I remember that from Chapter 7 page 5" for example

whew! In a nutshell, too many variables in light, atmosphere, frequency, altitude, weather etc etc to say NO.

Poor Matt is probably going to stay up now after this. Sorry Matt.

Chris

[circumpolar (Matt)]

Originally Posted by Zuts

I dont believe this applies to radio waves in the same way so i would say radio waves in the atmosphere are faster than light waves.



Cheers
Paul

Thanks for the response Paul.
You are right that the refractive index is different in different mediums but the question stated that the two (visiable light and radio waves, which are just light at a different energy level) are in the same medium.

The wiki you linked quoted this:
"The speed of light is of fundamental importance in physics. It is the speed of not just light, but of all electromagnetic radiation, as well as gravitational waves and anything having zero rest mass."

Radio waves are just another part of the Electromagnetic Spectrum and so have the same speed in the same medium. Me thinks.
The wiki doesn't directly address the issue.

I think I need some direct facts on the matter.
You know, so I can sleep at night.

[Screwdriverone (Chris)]

Damn, more googling has turned up something....

"Velocity of a radio wave that is radiated into space by a transmitting antenna is simply that speed at which the wave travels. Radio waves travel in free space at about speed of light and the waves traveling inside earth's atmosphere have lesser speed due to barometric pressure, humidity, molecular content and so on. The frequency of radio wave has nothing to do with the wave velocity."

Again, this means (or implies) that Radio waves SLOW down in the atmosphere too.... but.....does this then mean that the Speed of Light and the Speed of Radio waves in the atmosphere is therefore different? Ie not the SAME?

Yes, I would then say it is.

Therefore the answer to the question asking:
Q2) Is the speed of light and radio waves the same in the atmosphere?

I would say NO.

Does that logic seem right?

Chris

[circumpolar (Matt)]

Originally Posted by Screwdriverone

Hmmmmmm, interesting point Zuts, however, further down the page it says that;

"Since the speed of light in a material depends on the refractive index, and the refractive index may depend on the frequency of the light, light at different frequencies can travel at different speeds through the same material. This effect is called dispersion"

This statement is odd. I really don't like how they use the term "may depend" ??? This seems like a logical error!
There is no doubt that the fequencies are different as the wavelengths are too.

[circumpolar (Matt)]

Chris,
Are the websites your searching legit?
I 'm trying only to search university, astronomy and physics sites.
There's so wuch garbage out there.
Just try to pin down the range of the spectrum. You will get many results?!

[Screwdriverone (Chris)]

No, I see what you mean, but what that means is that the refractive index changes for light of different frequencies just like different frequencies of radio waves are affected differently in the atmosphere, eg. HF radio "skips" off the ionosphere and VHF radio is really only good up to about 5-10km line of sight.

I know I am drawing a pretty long bow when I compare light to radio frequencies like that but what I am getting at is that different frequencies are refracted differently in the medium, just like 1900Mhz doesnt like going through rolls of bubble wrap (happens to me at work using cordless DECT phones)

Have we slipped off topic?

ummm, now I cant sleep!

Chris

[Screwdriverone (Chris)]

yes, well, maybe, maybe not, on closer inspection....

http://www.articlegarden.com/Article...inition-/61655

It may not be the most definitive source, but it does validate my memory of the theory involved here.

Sorry if it has been misleading... you may be right of the source being as crap as wiki... after all, just cause its on the web doesnt make it true.

Chris

[circumpolar (Matt)]

Originally Posted by Screwdriverone

I know I am drawing a pretty long bow when I compare light to radio frequencies like that but what I am getting at is that different frequencies are refracted differently in the medium, just like 1900Mhz doesnt like going through rolls of bubble wrap (happens to me at work using cordless DECT phones)

I agree. They do refract differently.
But what about their speeds.

[MrB (Simon)]

Sorry, kinda off topic but kinda related, and you might find it pretty interesting is 'Slow Glass', a kind of picture window, read about it here: http://www.technovelgy.com/ct/content.asp?Bnum=692
or for a more technical(way over my head) explaination, do a search for "Bose-Einstein condensates"

[Zuts]

Hi,

I would say the speed of an electromagnetic wave NOT in a vacume does depend on the wavelength AND the refractive index. If this was not the case then there would be no rainbows as the different wavelengths would all have the same speed and so would not diverge.

Radio waves generally are thought of as electromagtnetic waves of enormous wavelengths (compared to light) they dont refract as much in a transparent medium such as air, water, glass, diamond and so would be faster than light in any given medium; EXCEPT in a vacume.

Cheers
Paul

[Screwdriverone (Chris)]

hmmm, yes.... well..... speeds.....ummmm.

Wouldnt the speed remain the same even if they were;

1) dispersed
2) refracted
3) impeded
4) anything else?

Fundamentally as the speed of light and the speed of radio waves is the same, we arent talking about the SPEED changing by stuffing it up in the atmosphere, the SPEED is still the same, certainly, does this mean that the light coming through a 90mm refractor telescope is "slowed down"?

I dont think that it matters whether the white light is refracted to green, diffracted, or scattered or blocked altogether, the SPEED of the wave remains the same!

So therefore, I go back to thinking you are right again.

I think....

Maybe...

Oh i dont know!

Chris

[Screwdriverone (Chris)]

Hi Paul,

Um, no, the rainbows (a la Pink Floyd album cover) are caused by the white light being split up into its components by the refraction of the light into its different frequencies or wavelengths not its different speeds.

The light may be "bent" by the refractive index of the medium, but this just means the light wave is "bounced" of something like a snooker ball on the side of the table, which means it takes LONGER to get there, because it went FURTHER, but the SPEED of the wave didnt change on the way.

ie, The time taken for green light and red light takes the same amount of time to reach your eye. It isnt slower because its a different colour just the number of humps in the wave is different.

Chris

[Zuts]

Hi,

Since the speed of light in a material depends on the refractive index, and the refractive index may depend on the frequency of the light, light at different frequencies can travel at different speeds through the same material. This effect is called dispersion.

Rainbows, see http://en.wikipedia.org/wiki/Speed_o...rent_materials

Cheers
Paul

[Astro78]

Q2. The reason light is slowed down in the atmosphere is the same as to why radio waves slow too (I think). Both slow down yes, but at the same speed, not sure they would.

What causes a photon to slow down? Isn't this caused by an oscillating EM field in the photon interacting with the electrons in the atmosphere?

If I am correct above, and since light has higher energy photons to that of radio waves - they cannot be traveling at the same speed with consistent torque being applies to both.

Wouldn't like to answer that with the clock ticking! Head hurts. Can a physicist please step in?

[Miaplacidus (Brian)]

Well, I don't know about Matt, but I'm pretty much put to sleep...

[circumpolar (Matt)]

I'm glad I have all the 'Astronomy Cast' episods recorded. Going to listen to 103,16,83 which are all about EM Spectrum. Will checkout the show notes and included links as well.

i just don't trust wiki when I need a specific answer.

Keep the thoughts comming.