The flashlight analogy is actually one that I not only described to people, but built a unique product around in the '90s. CyberFiber (yeah ... it was an tongue-in-cheek name for an engineering project that escaped into the marketing and sales department) was a duplex communication platform that used the same wave-length of light for both directions. It was a great marketing tool but required that your optical connections had very low reflectivity (and nobody likes to put coupling gel in their connectors).
With great difficulty.
That is one dimension of the "optical dielectric function" which is to do with the directionality of variation.
Light is the motion of magnetic and electric fields, the "ease" of this motion is affected by a variety of factors.
The speed of light depends not only on electric, but also magnetic field formation. And of each kind, there are a variety factors.
The vacuum itself resits electromagnetic motion to some degree. I was observing that it isnt the case that light simply has one speed, the field in which it travels itself is damped.
I guess you object to the "The speed of light, that is the speed of a photon, is a constant" part, where he identifies "light" with "photons".
In any case, I feel objections to what you said should come from a “better” QED description and not from a “lower-level” classical description.
Free space (vacuum) has an RI of 1.0. Photons move fastest in free space, about 300,000km/sec.
Water has an RI of ~1.33, meaning photons move through water at about 1/1.33, or about 75% of the free space velocity of light.
RI of optical fiber is often ~1.4. So while there may be path length increase due to off-axis propagation and bouncing, the actual photon speed in the medium is 1/1.4, or about 71% of the 300,000km/sec speed of light in vacuum.
Sure, as he said "Speed of light through glass is ~2e5 km/s"
> Water has an RI of ~1.33, meaning photons move through water at about 1/1.33,
Not really. Something moves at this lower speed, but it's not the photons.
Specifically with a classical E-M model, when you are in a material different than vacuum, the differing dielectric and permittivity of the material are (in a macroscopic sense) what lead to the reduced c*. This is encapsulated in the form of the index of refraction, which is effectively the ratio of velocities between vacuum and in-bulk.
At a microscopic E-M quantum level, you can argue about vacuum state differences between physical vacuum and in-bulk. Indeed, in a physical vacuum, you have virtual pair creation/destruction, virtual photons, etc. In bulk, you may have that, but you also have the bulk electronic and physical structure to work with, as well as different fields and field structures.
c is the speed of light in vacuum, a universal constant that obviously doesn't depend on the medium.
Edit: To be specific, in this context c was used for the speed of light in the medium and c_0 for the speed of light in vacuum. (To match epsilon and epsilon_0).
Edit: yes, I know. But using c in that way is absolutely non-standard in physics (v is used instead). Actually he later used c* for the speed of light in a medium, so he knows what c is.
https://en.wikipedia.org/wiki/Photon#Photons_in_matter
Light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. [...] The factor by which the speed is decreased is called the refractive index of the material.
In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and that new light interfering with the original light wave to form a delayed wave.
In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter to produce quasi-particles known as polariton (other quasi-particles are phonons and excitons); this polariton has a nonzero effective mass, which means that it cannot travel at c.
It's pretty common to hear people refer to the "speed-of-light in a vacuum". If the speed of light were a constant, why would you need to specify the vacuum? It's also worth noting that electrons don't move through conductors at the speed-of-light either - in the cable television industry, the best hard-line coaxial cable was about 0.85.
Speed of a photon = Speed of light in vacuum
The drift velocity of electrons through conductors is many orders of magnitude lower than the speed of light (~mm/s).
http://www.feynmanlectures.caltech.edu/I_31.html
"We have said before that light goes slower in water than in air, and slower, slightly, in air than in vacuum. This effect is described by the index of refraction $n$. Now we would like to understand how such a slower velocity could come about. In particular, we should try to see what the relation is to some physical assumptions, or statements, we made earlier, which were the following:
That the total electric field in any physical circumstance can always be represented by the sum of the fields from all the charges in the universe.
That the field from a single charge is given by its acceleration evaluated with a retardation at the speed $c$, always (for the radiation field).
But, for a piece of glass, you might think: “Oh, no, you should modify all this. You should say it is retarded at the speed $c/n$.” That, however, is not right, and we have to understand why it is not."
>If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.