How Does Light Speed Vary with Environment and Obstacles?

[eggman]

So how fast is light moving ?

...does atmosphere slow it down?

or when it deflects off and object does it slow down?

...does it slow over distance like a rifle bullet ?



If I walk into the back yard under a tree...how fast light be?

 

[eggman]

They can slow light according to wikapedia..(i just found out)

Why can't it..apparently being a 'thing' be captured and held onto ?

 

[Hootenanny]

Yes, the speed of light is reduced in the atmosphere. Reflection is slightly more complex, however it is simple enough to say that when the light ray leaves the mirror it is traveling at the same speed it 'struck' the mirror. No it doesn't slow over distance.

~H

 

[Hootenanny]

They can slow light according to wikapedia..(i just found out)

Congratulations, you have discovered the principle behind refraction.

~H

 

[eggman]

Thanks...wikapedia is good

 

[Hurkyl]

Light can be "captured" too. In some sense, that's exactly what's happening when you are warmed by sunlight! But they have done experiments where they capture light and then release it in roughly its original form.


But the words "captured" and "slow" are describing the macroscopic behaviors of light, and not individual photons which always travel at c throughout their entire existence.

 

[Hootenanny]

Further to what Hurkyl said. The observed speed of light in a medium is different to the actual speed of light. And to reiterate what Hurkly said, photons always travel at c throughout their lifetime. Wiki explains it in easy to understand terms;

Note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vacuum). On the microscopic scale, considering electromagnetic radiation to be like a particle, refraction is caused by continual absorption and re-emission (not necessarily in quite the same direction) of the photons that compose the light by the atoms or molecules through which it is passing. In some sense, the light itself travels only through the vacuum existing between these atoms, and is impeded by the atoms. The process of absorption and re-emission itself takes time thereby creating the impression that the light itself has undergone delay (i.e. loss of speed) between entry and exit from the medium in question. It may be noted, that once the light has emerged from the medium it changes back to its original speed and this is without gaining any energy. This can mean only one thing - that the light's speed itself was never altered in the first place. Alternatively, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.

So yes, light does change its speed when considered on the macroscopic scale, however, photons always travel at c. Therefore, the speed of photons is always constant.

~H

 

[eggman]

Where does light come from ?

What is its origin ?

it appears to be the constant...everything else is measured in relation to it..correct?

Light is a very fascinating subject...without it...nothing would be...

(no i am not a philosopher if u are wondering)

 

[eggman]

Is'nt light a mystery to a physicist?

...yeah...a mad rush to nowhere : )

 

[ZapperZ]

I know that in all the "excitement" of finding a new forum, you want to ask all of these questions. May I suggest you get use to how this forum is run first, and what have already been asked by looking through a few threads in several of our forums. You'll do yourself a lot of favor by discovering that a lot of what you want to know have already been answered.

Start first with our FAQ in the General Physics forum.

Zz.

 

[rcgldr]

Is there any evidence that light changes frequency over time, like the frequency getting lower over long periods of time (an optional explanation for red-shift of light from very far away stars)?

 

[eggman]

do u mean is light subject to a 'decay' ?

...thats a good question...if i interpret it correctly

 

[Lyuokdea]

Is there any evidence that light changes frequency over time, like the frequency getting lower over long periods of time (an optional explanation for red-shift of light from very far away stars)?

That would violate conservation of energy:

For a single photon:

E = hf = \frac{hc}{\lambda}

If the frequency or wavelength were to shift the amount of energy carried by the photon would change accordingly, and energy wouldn't be conserved.

~Lyuokdea

 

[kmarinas86]

That would violate conservation of energy:

For a single photon:

E = hf = \frac{hc}{\lambda}

If the frequency or wavelength were to shift the amount of energy carried by the photon would change accordingly, and energy wouldn't be conserved.

~Lyuokdea

This definition of energy does not take into account potential energy.

http://en.wikipedia.org/wiki/Conservation_of_energy

Total energy is conserved.

 

[Lyuokdea]

Good call, I didn't stop to think about the gravitational dependence of the question.

~Lyuokdea

 

[Mk]

I've been impressed several times by Lyuokdea's posts. Very good.