Jump to content

Speed of light as it enters vacuum from a dense medium.


Recommended Posts

We know that light slows down inside a dense medium like glass, water, etc. But the moment the light beam gets out of the dense medium to vacuum, it regains the original speed c instantaneously.

I know that waves doesn't require acceleration to travel in a particular speed but as light also behaves in particle nature ,shouldn't it obey the laws of particles too ?

Link to comment
Share on other sites

In the quantum description, photons don't slow down when they enter a dense medium. However, they are delayed by interacting with electrons in the material; in between those interactions, they travel at c.

 

There are some good lectures on QED by Richard Feynman online where he explains this very well.

Link to comment
Share on other sites

  • 3 weeks later...

In the quantum description, photons don't slow down when they enter a dense medium. However, they are delayed by interacting with electrons in the material; in between those interactions, they travel at c.

 

Just to remind that I (and others) disagree with that claim.

----------------------

 

The speed and the acceleration of a particle results from how the waves combine, for instance to produce a maximum of probability density in some region. I have no worry with the particle changing its speed abruptly.

 

In the case of the photon, one should (...I didn't) check what happens with its momentum between both media. For electrons in solids at semiconductor interfaces like heteroepitaxy makes, it's the momentum that has some conservation laws, not the speed.

Link to comment
Share on other sites

 

Just to remind that I (and others) disagree with that claim.

 

----------------------

 

The speed and the acceleration of a particle results from how the waves combine, for instance to produce a maximum of probability density in some region. I have no worry with the particle changing its speed abruptly.

 

In the case of the photon, one should (...I didn't) check what happens with its momentum between both media. For electrons in solids at semiconductor interfaces like heteroepitaxy makes, it's the momentum that has some conservation laws, not the speed.

I don't know enough to actually understand the second paragraph so maybe the answer to the questions I am going to ask are in the second paragraph.

 

Where does the heat come from?

 

Wouldn't there be an uneven distribution of heat in the medium if the photon slowed down?

 

One more question if the photon slows down as it enters the medium would the result be a change in density where it enters the medium that would cause the next photon to slow even more, then the next to slow even more etc. To the point where photons can no longer enter the medium?

Edited by jajrussel
Link to comment
Share on other sites

Where does the heat come from?

 

You mean when a material absorbs photons?

 

The momentum of the absorbed photons is transferred to the atoms that absorb them. This gives the atoms a "kick" and sets them moving (more than they were before). "Heat" is just a measure of the average kinetic energy of the particles in the medium - the atoms now have more kinetic energy, hence more heat.

 

Wouldn't there be an uneven distribution of heat in the medium if the photon slowed down?

 

There is: the photons are (in general) more likely to be absorbed near the surface so the surface will be warmer.

 

One more question if the photon slows down as it enters the medium

 

It doesn't.

Link to comment
Share on other sites

  • 2 weeks later...

Yes it does. A photon in glass is slower than in vacuum. You can really forget and erase from memory that explanation by the bizarre book. It is disproven by observation and it results from misconceptions about QM.

 

Observation:

  • Materials have a permittivity even at DC, and when no process slows down the permittivity, the speed of EM waves results directly from this permittivity. For many materials, the MHz and GHz index is exactly the square root of the DC permittivity; for silicon it's accurate up to the near infrared. No delay involved here.
  • If "the atoms" were briefly excited, we would measure a photocurrent when light passes through glass.

Misconceptions:

  • An interaction, for instance photon with electron, does not happen at a point. It happens over all the volume common to both particles, which can be the volume of an atom, or of many atoms in a solid. It is even computed that way.
  • There is no vacuum between the atoms of a crystal. The valence electrons take all the volume.
  • The valence electrons spread over many atoms in a solid. The always filled states are as big as the solid itself; near the Fermi level, we may define an electron extension differently, from its mean thermal energy for instance, and then it spans over thousands of atoms, and so does the interaction.
  • Photons spread over many atomic distances, so their propagation "between the atoms" is nonsense.
  • The absorption or emission of a photon is a slow process. If it takes 1ns for instance (the transition duration between the states, also the light's coherence duration, and related with the line bandwidth) then the photon spans over 2 billion atoms, so the flight time between two atoms makes little sense.
  • One cannot measure a photon propagation time that is much smaller than one light period, and far less so if the photon has a limited bandwidth as usual.
  • Distinguishing light from photons is a bad idea. All the information, everything we can observe is in the wavefunction.

This is not just a matter of concepts. In semiconductors, quantum wells, quantum dots are accurately designed and computed with electrons spanning many atoms, and the energy levels match the observed optical properties. Photon absorption and emission happens over this whole extension, generally tens of atoms thick, because an electron more localized after the interaction would give completely different results. Even the direction of polarization of the photon fits the interference of the electron wavefunctions of both eigenstates - it wouldn't be the case with an interaction limited to one atiom or to a point.


[...]

 

Wouldn't there be an uneven distribution of heat in the medium if the photon slowed down?

One more question if the photon slows down as it enters the medium would the result be a change in density where it enters the medium that would cause the next photon to slow even more, then the next to slow even more etc. To the point where photons can no longer enter the medium?

 

Energy is conserved when all light enters a transparent material (say, with an antireflective coating). A slower speed doesn't mean less energy per photon, which depends just on the frequency.

 

In case of doubt: don't compare photons with bullets whose kinetic energy relies on mass and speed. The energy of light corresponds to electric and magnetic fields; in a medium with permittivity, the electric field is smaller but the polarization bigger, resulting in the same energy.

 

At a slower speed, the same number of photons per second results in more photons per metre. No worry with that, except that photons normally aren't counted on their way. No accumulation neither. Anyway, photons interact very little under normal circumstances, so they can accumulate without consequence.

Edited by Enthalpy
Link to comment
Share on other sites

Yes it does. A photon in glass is slower than in vacuum. You can really forget and erase from memory that explanation by the bizarre book.

 

That "bizarre book" was written by one of the key developers of QED. So I think he understands the theory pretty well.

 

 

It is disproven by observation and it results from misconceptions about QM.

 

If QED had been disproved, I think it would have made the headlines. And there would have been Nobel Prizes.

 

Although, to be fair, my (limited) understanding of QED is that not only does every possible path of the photon have to be taken into, but also every possible speed (including faster than light).

Edited by Strange
Link to comment
Share on other sites

 

Observation:

  • Materials have a permittivity even at DC, and when no process slows down the permittivity, the speed of EM waves results directly from this permittivity. For many materials, the MHz and GHz index is exactly the square root of the DC permittivity; for silicon it's accurate up to the near infrared. No delay involved here.
  • If "the atoms" were briefly excited, we would measure a photocurrent when light passes through glass.

 

 

Speed of EM waves ≠ speed of photons

 

From where would this alleged photocurrent come?

 

Misconceptions:

  • An interaction, for instance photon with electron, does not happen at a point. It happens over all the volume common to both particles, which can be the volume of an atom, or of many atoms in a solid. It is even computed that way.
  • There is no vacuum between the atoms of a crystal. The valence electrons take all the volume.
  • The valence electrons spread over many atoms in a solid. The always filled states are as big as the solid itself; near the Fermi level, we may define an electron extension differently, from its mean thermal energy for instance, and then it spans over thousands of atoms, and so does the interaction.
  • Photons spread over many atomic distances, so their propagation "between the atoms" is nonsense.
  • The absorption or emission of a photon is a slow process. If it takes 1ns for instance (the transition duration between the states, also the light's coherence duration, and related with the line bandwidth) then the photon spans over 2 billion atoms, so the flight time between two atoms makes little sense.
  • One cannot measure a photon propagation time that is much smaller than one light period, and far less so if the photon has a limited bandwidth as usual.
  • Distinguishing light from photons is a bad idea. All the information, everything we can observe is in the wavefunction.

 

If the electron is spread out as you describe, how can you have a dipole moment?

 

An absorption that results in the electron being excited to a real state is "slow" but what about a virtual-state absorption?

Link to comment
Share on other sites

Yes it does. A photon in glass is slower than in vacuum. You can really forget and erase from memory that explanation by the bizarre book. It is disproven by observation and it results from misconceptions about QM.

How was a photon observed to be moving slower without observing its behaviour?

 

Saying it slows down because the photon can be treated as a continuous classical wave interacting with a not very classical continuous electron wave can give correct answers to a high degree of accuracy.

No doubt this is one reason Newton's belief that light was composed of particles was rejected for some centuries.

An interaction, for instance photon with electron, does not happen at a point.

Zap your glass with gamma rays and as you increase the energy you can make each interaction occur in an arbitrarily small volume. (The gamma ray is the pre QM 'passive observer').

 

Has QM proven that photons travel at variable speed? :confused:

 

Distinguishing light from photons is a bad idea. All the information, everything we can observe is in the wavefunction.

This is not just a matter of concepts. In semiconductors, quantum wells, quantum dots are accurately designed and computed with electrons spanning many atoms, and the energy levels match the observed optical properties. Photon absorption and emission happens over this whole extension, generally tens of atoms thick, because an electron more localized after the interaction would give completely different results. Even the direction of polarization of the photon fits the interference of the electron wavefunctions of both eigenstates - it wouldn't be the case with an interaction limited to one atiom or to a point.

You may be thinking of observing these photons using a short wavelength low energy electromagnetic field. Einstein's Nobel prizewinning paper on the photoelectric effect demonstrated you can't do this.

The wavefunction of the field of the photoelectric photon could be 1/100th mm or a mile in diameter before the interaction. No way to tell and this experiment cannot be explained if light is not quantised.

 

 

 

Using electron(s) in a double slit experiment you see an interference pattern provide you don't observe the interaction.

Similarly you can calculate that eg a photon has a 10% chance of interacting with a field a ( or do you mean particle a?)

and 3% with field b. IF you observe with a photon or particle you will affect it so much that you could say it has eg a 99.99% chance of interacting with a and a 3% (or so) chance of interacting with b.

 

ie interactions do not localise particles until they are observed.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.