Exam Question Today

[circumpolar]

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:help:
PS. If you know a good reference please include it.

[Screwdriverone]

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/astronomy/q0254.shtml

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

Cheers :thumbsup:

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" :lol:

[circumpolar]

:lol: 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 :rolleyes:.

[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]

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]

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 (http://en.wikipedia.org/wiki/Electromagnetic_radiation), as well as gravitational waves (http://en.wikipedia.org/wiki/Gravitational_wave) and anything having zero rest mass (http://en.wikipedia.org/wiki/Invariant_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.:rolleyes:

[Screwdriverone]

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]

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]

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?!:doh:

[Screwdriverone]

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]

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

http://www.articlegarden.com/Article/The-Properties-of-Radio-Waves--The-Simple-Definition-/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]

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

[MrB]

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]

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]

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_of_light#Interaction_with_tra nsparent_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]

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

[circumpolar]

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.

[sjastro]

Q1 Yes. Q2 No.

And perhaps suprisingly radio waves will travel faster in the atmosphere compared to light.

It's based on Rayleigh scattering where scattering of a photon is a function of its wavelength. The shorter the wavelength, the greater the scattering and energy loss of the photon by atmospheric molecules.

Radio waves have much longer wavelengths than light.

It explains why the sky is blue.

Regards

Steven

[Astro78]

Couldn't agree more

[Astro78]

+ See Raman scattering

[sjastro]

You're right I should have referred to Raman scattering.

Regards

Steven

[circumpolar]

I agree that the two photons (visable light & radio) will reach point 'B' at different times in a given medium, with the radio arriving first, but that seems to me to be a result of their different paths traversed through that medium, which is caused by their respective wavelengths.
What I'm trying to determine is their speed in that medium, not the time difference between entering and exiting the medium.

Given the equation 'Wavelength x Frequence = C'
It seems to me that the refractive index of a given medium does not come into it when we are comparing their relative speed.

That is,
Wavelength(light) x Freq(light) = C
Wavelength(radio) x Freq(radio) = C

As you can see it is the product of each Wave & Freq that will always adjust itself to equal the SPEED of C. All the numbers can change except C.

[circumpolar]

I found this responce to a similar question at physlink.com
http://www.physlink.com/Education/AskExperts/ae27.cfm


"All electromagnetic radiation, of which radio waves and X-rays are examples, travels at the speed c in a vacuum. The only difference between the two is that the frequency of X-rays is very much higher than radio waves.

If, on the other hand, the radio waves or X-rays are propagating through a medium other than the vacuum, their speed will generally be less than c and will depend upon the specific properties of the medium. Thus, in principle, neither can be said definitely to travel faster than the other if the medium is other than the vacuum."
Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA

So nether can be said to travel faster then the other in a medium.
I think the last sentence supports me. :)

[Astro78]

This Dr. Davis may be right, but, how he derives at "THUS...." just aint cutting the mustard. Too limited in his derivation.

ps: he is the man to contact if needing expert testimony if you've been trapped in an automatic door, seriously, check it out http://www.davis-inc.com/ :P

pps: very interesting - http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html

[sjastro]

Your conclusion doesn't make any sense. I think you are trying to equate phase velocity of a wave with the velocity of a photon.

First of all it's Wavelength X Wave number = C.

Wave number is simply another way of expressing wavelength (C/wavelength).

I can measure a sound wave's wavelength and wave number, multiply the two together and get C. Does that mean the speed of sound = C? No.

The energy of photon is given E=hv. h is Plank's Constant, v is frequency.

When a photon is inelastically scattered it can lose energy, and the frequency is decreased. Since it is a free photon the it will lose kinetic energy or in other words it will lose speed.

Regards

Steven

[sjastro]

But we do know the properties of the medium and how photons of different wavelengths react in that medium.

Regards

Steven

[AGarvin]

Are we talking the speed of light here or the velocity of the photon? The speed of light in a given medium is the same regardless of wavelength.

The EM formula is f = c/l where

f = frequency
c = speed of light (of course)
l = wavelength

If you double the frequency, you halve the wavelength, so c remains constant.

Andrew.

[circumpolar]

I got this equation from 'Astronomy Cast ep.16_Across the Electromagnetic Spectrum'.
I admit it was only spoken and I have infered.:whistle:


Here is a direct quote from Dr Pamela Gay off the transcript page.

"So when we're talking about the colour of something, we can use a lot of different terms. We can say that it has a wavelength of 14.48 GHz, that's formaldehyde. Or we can say it has a wavelength of 21cm. You get these two numbers because wavelength times frequency just happens to work out (because of the way the Universe was formed) to be the speed of light."
http://www.astronomycast.com/astronomy/episode-16-across-the-electromagnetic-spectrum/

:shrug:

[circumpolar]

This must be what Dr Pamlea Gay was talking about.
Thanks Andrew :thumbsup:

[sjastro]

Sorry if I came over a bit abrupt.

Steven

[sjastro]

Andrew,

The formula is only applicable for a vacuum. If a photon does not lose energy in a medium, the frequency remains constant. The photon can still slow down as the energy is independant of velocity (= Planks constant X frequency).

This can only occur if the wavelength changes (but frequency remains the same).

Regards

Steven

[BerrieK]

Umm...I thought Raleigh scattering was coherent scattering where the incident photon undergoes a change of direction without a loss of energy or change in wavelength..my understanding was that Compton scatter involves the incident photon changing direction and imparting some of its energy to a recoil electron, hence changing the wavelength of the incident photon.

[AGarvin]

Good point :doh:.

[xelasnave]

This may be of interest and slime relevence

http://www.sciencedaily.com/releases/2007/08/070822164415.htm

alex

[g__day]

I would have said yes to both questions for an under grad course. E/M radiation will be affected by an atomsphere that can absorb and emit the wave - so propogration speed could vary in a non linear fashion - but that is kinda a very advanced view of a very subtle shift.

An aside - even though electrons travel through a wire at pretty much light speed (individually) en-masse electron drift is about 10% of lightspeed - due to collisions and a network haze of traffic though a metallyic crystaline lattice.

An individual quanta of E/M radiation at any particular frequency hitting an atomsphere will be absorbed and re-emitted by its atoms. The duration of absorption and re-emittance and how that might shift with the frequency or energy of the radiation hitting the atom is fairly advanced physics - not typically high school or under grad Uni physics I'd imagine? If the question is wanting you to know will high energy e/m radiation be absorbed and re-emitted by a gas faster or slower than lower energy e/m - or secondly have statistically more or fewer collisions - that is a pretty advanced question.

I'd treat any e/m radiation as travelling though a consistent media at the same pace, unless your radiation has a energy level so high that it starts to warp spacetime. So yes - shoot say a 10 ^ 80 Joule cosmic ray into the atomsphere and it will curve spacetime with its passage - but that isn't the pretext of the question.

[circumpolar]

I get the feeling that the question was poorly written.
I think it was ment to prompt us to identify that there is a difference in total travel time between two points when you compare the two beams. That is, each beam is traveling at the same speed within the medium, but traverse a different pathway due to refraction, resulting in different time intervals.

[sjastro]

Refraction only occurs at the interface of two mediums. There is no deviation in the medium.

Apart from Raman scattering slowing down visible light more than radio waves, there is another consideration.

Low frequency radio waves have wavelengths in the range of 1000-100,000 km, the troposphere is only about 20 km thick. Only a small percentage of the wavelength actually interacts with the bulk of the atmosphere unlike the wavelength of light.

Regards

Steven

[Phoenix]

My two cents worth - I believe Steven has hit the nail on the head here. The lower frequency radio waves have much longer wavelengths than visible light and thus visible light will interact considerably more than radio waves through the same medium. Consequently, the velocity of visible light will be marginally slower (through a vacuum the speed is pretty much the same). I understand this interaction with the atmosphere to be associated with Rayleigh's scattering, Mie scattering and Non-selective scattering of visible light.

Cheers
Steve

[circumpolar]

Even though it occurs at the interface, the event causes the beams to take different pathways (vector) through the medium.

Good point regarding the very long radio waves. :thumbsup: Once again this demostrates the unspecific nature of the Question.

As a side note, I have since learnt that it is the Frecency that is least affected (if at all) when transitioning through media. That is, the speed is changed (Cm = C / refractive index) and the wavelength (lamda) also changes proportionaly with Cm to concerve the Frequency value.
This way the speed is always equal to the product of Frequency & Wavelength. ;)

[circumpolar]

Agree.
That is, When we are talking about velocity I agree.
But remember we are talking about speed here, wich has a specific meaning.

[circumpolar]

I should get my exam marks back today, unless there is some sort of teacher strike. :whistle:

I'm now ready to fight for my interpritation of the question.
Thanks for all your help in nutting this out.:hi:
Who knows, I may have given the correct answer after all.:lol:

[sjastro]

Velocity is speed in a particular direction. In reference to your question the terms are interchangeable.

If the question is simply asking is there a difference in the speed (velocity) of light, between light and radio waves in vacuum, and between light and radio waves in air, then refraction, different trajectories in a the medium etc are not relevant.

Seems like the question has been made unnecessarily complicated.

Regards

Steven