Exam Question Today

[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]

Originally Posted by circumpolar

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

Couldn't agree more

[Astro78]

Originally Posted by sjastro

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.

+ See Raman scattering

[sjastro]

Originally Posted by Astro78

+ See Raman scattering

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

Regards

Steven

[circumpolar (Matt)]

Originally Posted by 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

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 (Matt)]

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]

Originally Posted by circumpolar

"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

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/

pps: very interesting - http://www.hno.harvard.edu/gazette/2...stoplight.html

[sjastro]

Originally Posted by 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.

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]

Originally Posted by 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.

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 (Matt)]

Originally Posted by 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

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


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/astrono...etic-spectrum/

[circumpolar (Matt)]

Originally Posted by 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.

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

[sjastro]

Originally Posted by circumpolar

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


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/astrono...etic-spectrum/

Sorry if I came over a bit abrupt.

Steven

[sjastro]

Originally Posted by AGarvin

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.

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]

Originally Posted by sjastro
The formula is only applicable for a vacuum.

Good point .

[xelasnave]

This may be of interest and slime relevence

http://www.sciencedaily.com/releases...0822164415.htm

alex

[g__day (Matthew)]

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 (Matt)]

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]

Originally Posted by 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.

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