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Velocity of light


Craig Dilworth
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Light does change speed within a medium, and that does generally vary with the wavelength. This is known as dispersion. v = c/n, where n is the index of refraction (n=1 for a vacuum as you might suspect from the previous answers)

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Light's color varies depending on the frequency of the wave, the constant velocity doesn't change with color.

 

Thought I would add this; a simple expression to help swallow the concept:

 

[math] f = \frac{c}{\lambda} [/math]

 

Where "c" is the speed of light, [math] \lambda [/math] is the frequency of the light, and "f" is the frequency. This only works in a vacuum because light doesn't have a average velocity of "c" in different media as Swansont stated.

Edited by mississippichem
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How can the velocity of light be slower or faster depending on colour (n) and at the same time have a constant vacuum velocity of c? (Answers presupposing special or general relativity not of interest.)

 

Are you thinking about red shift and blue shift, if you are then it depends on the speed of the object emmiting or recieving the light not the speed of light itself (which is always constant). If the speed of different colours of light were different then you would see this during an eclipse

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the velocity of light does not change with colour. i don't know where you heard it but it isn't true.

 

Of course the velocity of light changes with colour. I don't know where you heard otherwise (since you haven't told me), but it isn't true.

 

If we start with the basic equation v = λn, then with a Doppler effect we get a change in n (fcy/colour), which, given constant λ (wavelength), implies a change in v. Or is v = λn no longer viable?

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ah, you're using the equation wrong. frequency and wavelength are variables the velocity is not. you can't have a constant wavelength and varying frequency.

 

pretty much all physics texts and experiments confirm the speed is constant with relation to colour. the only ones that don't are those unrelated to the topic of light.

Edited by insane_alien
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Thought I would add this; a simple expression to help swallow the concept:

 

[math] f = \frac{c}{\lambda} [/math]

 

Where "c" is the speed of light, [math] \lambda [/math] is the frequency of the light, and "f" is the frequency. This only works in a vacuum because light doesn't have a average velocity of "c" in different media as Swansont stated.

 

 

 

To start, let's not use c, but v, since c, being a constant, begs the question. Then we get f = v/λ, or, in the notation I'm familiar with, n = v/λ, or v = λn. Thus λ is not, as I'm sure you didn't mean it to be, fcy (colour); n (= f) is fcy (= colour). But then my comments to other contributors should apply.

 

 

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ah, you're using the equation wrong. frequency and wavelength are variables the velocity is not. you can't have a constant wavelength and varying frequency.

 

pretty much all physics texts and experiments confirm the speed is constant with relation to colour. the only ones that don't are those unrelated to the topic of light.

 

 

No, I should say, fcy and vel are variables; wavelength is not. At least this is what I think I've leared from my study of the subject. Cf e.g Otis (1963), p. 10. The motion of a spectroscope towards or away from a star, caused by the orbital and rotational motions of the earth, cannot in any way affect the ‘wavelength’ (λ) of the light coming from the star. This means that when the motion of the earth causes the spectroscope to approach a star, the shift of the spectrum of the star towards the violet clearly indicates an increase in the frequency (n) of the reception of the constituents of the starlight by the spectroscope. And a red-shift when the spectroscope recedes from the star clearly indicates a decrease in the frequency.

“Since velocity equals wavelength multiplied by frequency (v = λn), it follows that when the wavelength of the light coming from a star is unchanged [= the light coming from the star doesn’t change], and its frequency of reception by the spectroscope changes, as indicated by the shift of the spectrum of the light, the velocity (λn) of the light relative to the spectroscope changes.” And it follows further that, more particularly, it is the frequency of the radiation that changes, given that the wavelength of the light emitted is constant, as in the above example.

 

Otis (1963), p. 13. According to the light postulate there is no way by which an observer can detect any difference in the velocity of light relative to him. [so, given the special theory, the postulate that the speed of light is constant cannot be checked?] Nevertheless we see that the shift of the spectrum of light when the spectroscope is moved towards the source provides us with definite empirical evidence that the frequency of reception of the wavefronts by the spectroscope is increased, and hence (l remaining unchanged) the velocity of the light relative to the spectroscope (and relative to the laboratory and observer) is increased, thus contradicting the light postulate.

 

 

 

 

 

 

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Of course the velocity of light changes with colour. I don't know where you heard otherwise (since you haven't told me), but it isn't true.

 

If we start with the basic equation v = λn, then with a Doppler effect we get a change in n (fcy/colour), which, given constant λ (wavelength), implies a change in v. Or is v = λn no longer viable?

 

No. I'm not sure where you heard this, but it's not true. In a vacuum the speed of light is a constant, which is a ramification of Maxwell's equations. If it weren't your car radio wouldn't work. The Doppler effect changes both frequency and wavelength in inverse relation, so that the speed remains constant.

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To start, let's not use c, but v, since c, being a constant, begs the question. Then we get f = v/λ, or, in the notation I'm familiar with, n = v/λ, or v = λn. Thus λ is not, as I'm sure you didn't mean it to be, fcy (colour); n (= f) is fcy (= colour). But then my comments to other contributors should apply.

 

Color can be thought of as frequency or wavelength. They are really just different expressions of the same thing because light always travels at c in a vacuum. One just has units of length while the other has units of count/time.

 

If c isn't constant in a vacuum, the majority of modern physics would have to be overturned.

Edited by mississippichem
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The question is whether it is or not. It's not a matter of what either of us say.

 

Actually yes it is, when one is drawing on more than a hundred years' worth of research that depends on whether it's true or not. As mississippichem has stated, you would rewrite a lot of physics, and you'd be in the mystifying state of having things like GPS working, and yet the theoretical basis for its operation being wrong.

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With energy/photons there is a component that remains at C. We also have two finite components that we call wavelength and frequency. When light changes color, only these two finite aspects of the photon will change, but the C aspect will not be effected by the color change. Picture C as the root of a photon. Its two finite branches can move, but the root stays fixed. C is sort of an anchor state, with photons not able to move plus or minus C and still remain photons.

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it has been experimentally shown to be constant time and time again. the literature on the subject is overwhelmingly in support of a constant speed of light.

 

This reply begs the question. In the present context I would benefit most if you directed your comments to the reasoning involved in my Otis reference. Thanks!

 

With energy/photons there is a component that remains at C. We also have two finite components that we call wavelength and frequency. When light changes color, only these two finite aspects of the photon will change, but the C aspect will not be effected by the color change. Picture C as the root of a photon. Its two finite branches can move, but the root stays fixed. C is sort of an anchor state, with photons not able to move plus or minus C and still remain photons.

 

Could you please meet the line of thinking involved in my reference to Otis? Thanks! (What you present here just side-steps the issue.)

 

it has been experimentally shown to be constant time and time again. the literature on the subject is overwhelmingly in support of a constant speed of light.

 

Sorry, this isn't an argument, only hand-waving.

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Actually yes it is, when one is drawing on more than a hundred years' worth of research that depends on whether it's true or not. As mississippichem has stated, you would rewrite a lot of physics, and you'd be in the mystifying state of having things like GPS working, and yet the theoretical basis for its operation being wrong.

 

Yes, it presents a conundrum. But I'd appreciate your showing how Otis' (and and at least one so-far unmentioned other's) reasoning on this point is mistaken, rather than just referring to "a hundred years of research." Modern physics is a very tricky business, particularly since physicists now seem to accept such things as (Maxwellian, Lorentzian, Hertzian) electrodynamics (your GSP reference), which presupposes the existence of waves in a medium, and at the same time special relativity, which excludes the medium, as well as both the special and general theories of relativity, which are incompatible. By the way, by advocating the constancy of the speed of light you're denying the viability of the general theory of relativity, according to which the speed of light is variable. I think that much of this confusion stems from physicists' inclination to apply either wave or particle thinking whenever convenient (as QM has institutionalised).

 

Your suggestion of re-writing a lot of physics is interesting, since that's precisely what I'm engaged in!

 

As regards the GPS business, of course, logically, the results of Maxwellian electrodynamics could well be right while the theory itself is wrong.

 

Empirical demonstration is "begging the question?"

 

Could you please reply to the line of reasoning in my reference to Otis? Thanks!

 

Color can be thought of as frequency or wavelength. They are really just different expressions of the same thing because light always travels at c in a vacuum. One just has units of length while the other has units of count/time.

 

If c isn't constant in a vacuum, the majority of modern physics would have to be overturned.

 

You've got it!

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This reply begs the question. In the present context I would benefit most if you directed your comments to the reasoning involved in my Otis reference. Thanks!

 

This reference: Arthur Sinton Otis, Ph.D, "Light velocity and relativity: the problem of light velocity, disproof of the Einstein postulate" ?

 

Two words: Crack Pot. He wasn't even a physicist, and had no advanced training in physics. Otis was a psychiatrist or psychologist who apparently designed a school aptitude test. In his own field he may have been very good, but not in physics.

 

 

it has been experimentally shown to be constant time and time again. the literature on the subject is overwhelmingly in support of a constant speed of light.

 

Sorry' date=' this isn't an argument, only hand-waving.

[/quote']

No, claiming that experiment after experiment after experiment after experiment has shown that light does travel at c in vacuum regardless of the relative velocity between the source and the receiver and regardless of the frequency of the light is not hand-waving.

 

Word to the wise: Stop reading crackpots and start reading about the physics of relativity.

 

 

 

Edited by D H
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Are you thinking about red shift and blue shift, if you are then it depends on the speed of the object emmiting or recieving the light not the speed of light itself (which is always constant). If the speed of different colours of light were different then you would see this during an eclipse

 

When it comes to red- and blue-shift, you have to consider whether it is being understood in terms of a wave or particle theory. On a wave theory, such as Maxwell's, motion of the source is irrelevant. In the case of particle (electrodynamically relativistic) theories, the motion of the source is relevant, but only after the time it takes the radiation to reach the receiver.

 

As I mentioned to Swansont, that the vel. of light should be constant contravenes the general theory of relativity.

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Otis's work - some of which can be found on questia (I won't post link due to possible copyright problems) - directly contradicts maxwell, einstein etc. many seemingly consistent theories can be posited, however a constant speed of light which follows from maxwell's equations and is the basis of special relativity is the theory which matches experimental data. special and general relativity have been tested to an enormous extent, its predictions work, and real-world applications rely on the equations and physics it generates. From my very brief reading Otis claims that light does not have constant speed regardless of the motion of the observer/source - SR is based on the fact that it does. One is right, the other is wrong; I don't know if Otis's work can be mathematically self-consistent (he was/is a far greater mathematician than I) but even presuming that it is self-consistent it doesn't comply with known experimental results.

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This reference: Arthur Sinton Otis, Ph.D, "Light velocity and relativity: the problem of light velocity, disproof of the Einstein postulate" ?

 

Two words: Crack Pot. He wasn't even a physicist, and had no advanced training in physics. Otis was a psychiatrist or psychologist who apparently designed a school aptitude test. In his own field he may have been very good, but not in physics.

 

 

 

No, claiming that experiment after experiment after experiment after experiment has shown that light does travel at c in vacuum regardless of the relative velocity between the source and the receiver and regardless of the frequency of the light is not hand-waving.

 

Word to the wise: Stop reading crackpots and start reading about the physics of relativity.

 

 

 

 

 

Sorry, this is an ad hominem argument, which doesn't add to the discussion. Otis could be a mystic for all it matters. What you have to do is deal with what he says. And saying "experiment" four times doesn't strengthen your argument. But I'm curious. Which experiments do you have in mind, and do they presuppose a wave theory, or a particle theory, or neither?

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