[Bored chemist]

If I make an optical fibre out of glass with a refractive index of 1.5 and I send a flash of light down a kilometer of this cable, how long does it take the flash to reach the other end?
What is the speed of light in that glass?

If you use one of these sorts of things to measure the depth of a swimming pool, do you get the right answer?
https://www.toolstation.com/dewalt-dwht77100-xj-laser-distance-measure-30m/p67627?store=AG&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&mkwid=_dc&pcrid=&pkw=&pmt=&gclid=Cj0KCQjwl92XBhC7ARIsAHLl9ak1eLlUzABZzcj2kekc_LMN7C-ixh7mcv0xwXPE-UhtrNAlFav1mK0aAoEFEALw_wcB&gclsrc=aw.ds

[Origin]

It doesn't tell the mechanism for how the medium can change the light speed there. The last video I posted shows why it's false, using the refraction of X-ray from air to glass.
The first videos in this thread shows more examples. Have you watched them?

I suggest you figure out a way to get a physics degree.  You seem truly interested in physics but watching youtubes and trying to make experiments with subpar equipment is only confusing you.  Without a solid understanding of basic physics you will never understand the more complicated and nuanced concepts.

[hamdani yusuf (OP)]

If I make an optical fibre out of glass with a refractive index of 1.5 and I send a flash of light down a kilometer of this cable, how long does it take the flash to reach the other end?
What is the speed of light in that glass?

1.5 times the light needs to travel 1 km in vacuum.
2c/3.

If you use one of these sorts of things to measure the depth of a swimming pool, do you get the right answer?
https://www.toolstation.com/dewalt-dwht77100-xj-laser-distance-measure-30m/p67627?store=AG&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&mkwid=_dc&pcrid=&pkw=&pmt=&gclid=Cj0KCQjwl92XBhC7ARIsAHLl9ak1eLlUzABZzcj2kekc_LMN7C-ixh7mcv0xwXPE-UhtrNAlFav1mK0aAoEFEALw_wcB&gclsrc=aw.ds

No.

[hamdani yusuf (OP)]

It doesn't tell the mechanism for how the medium can change the light speed there. The last video I posted shows why it's false, using the refraction of X-ray from air to glass.
The first videos in this thread shows more examples. Have you watched them?

I suggest you figure out a way to get a physics degree.  You seem truly interested in physics but watching youtubes and trying to make experiments with subpar equipment is only confusing you.  Without a solid understanding of basic physics you will never understand the more complicated and nuanced concepts.

Questioning discrepancy between theory predictions and observation can be done anytime by anyone. No degree is needed.
Do you think X-ray travels faster through glass than air?
I pointed that the explanation in the video still contains discrepancies.

The explanation offered in this video is more compatible with my own experiments using microwave transceiver, although there are still some discrepancies.

[Bored chemist]

Are you aware that EM radiation has two velocities?
https://en.wikipedia.org/wiki/Phase_velocity
https://en.wikipedia.org/wiki/Group_velocity

[Bored chemist]

Questioning discrepancy between theory predictions and observation can be done anytime by anyone. No degree is needed.

No, but knowledge- perhaps acquired as part of a degree- is needed to understand those theories and see why there isn't really a discrepancy.

[hamdani yusuf (OP)]

Are you aware that EM radiation has two velocities?
https://en.wikipedia.org/wiki/Phase_velocity
https://en.wikipedia.org/wiki/Group_velocity

Yes. The question is, how do various matters change them differently?

[alancalverd]

I think confusion has arisen in several minds by failing to distinguish between speed and velocity.

The path of a photon is the direction of its velocity vector. Its speed depends on Maxwell's equation v = 1/√εμ (note v is a scalar) and in free space c = v₀ in all directions

For visible photons incident on a medium, v (vector) tends towards the normal, and as propagation depends on collective electron movements within the material, the refractive index generally increases with increasing photon energy (dispersion).  But as Deecart has pointed out,  ε < 1 at very high frequencies so the propagation direction can tend away from the normal, depending on the propagation mode, even if v (scalar) < c. 

Interesting? Only that the phenomenon was completely described by a Peterhouse man 40 years before x-rays were discovered.

[alancalverd]

PS it just occurred to me that you could look at x-ray refraction in terms of momentum transfer. Visible light just wiggles the electrons a bit so the wavelet analysis gives you a reasonable model of forward propagation, but x-ray photons transfer significant momentum, so conservation demands that the forward displacement of the substrate electrons is compensated by a lateral deflection of the propagation vector.     

[hamdani yusuf (OP)]

But as Deecart has pointed out,  ε < 1 at very high frequencies so the propagation direction can tend away from the normal, depending on the propagation mode, even if v (scalar) < c.

I guess you mean n<1.

The question is, how do various matters change light velocities differently?

[hamdani yusuf (OP)]

Interesting? Only that the phenomenon was completely described by a Peterhouse man 40 years before x-rays were discovered.

Why it's not widely acknowledged by physicists, as pointed out by the first videos here?

[hamdani yusuf (OP)]

PS it just occurred to me that you could look at x-ray refraction in terms of momentum transfer. Visible light just wiggles the electrons a bit so the wavelet analysis gives you a reasonable model of forward propagation, but x-ray photons transfer significant momentum, so conservation demands that the forward displacement of the substrate electrons is compensated by a lateral deflection of the propagation vector.     

It sounds like an ad hoc explanation to me. How can it be used to explain other closely related phenomena, such as reflection, total internal reflection, absorption, and polarization?

[alancalverd]

n is dependent on the more fundamental property, ε.

[alancalverd]

Interesting? Only that the phenomenon was completely described by a Peterhouse man 40 years before x-rays were discovered.

Why it's not widely acknowledged by physicists, as pointed out by the first videos here?

You will have to ask them.

Lack of a proper education, perhaps? Failure to respect true genius? Political Correctness (Peterhouse was one of the last colleges to admit women)? Maxwell was considered a bit odd, socially, at the time.

More likely: Few people study both classical and x-ray optics.The refraction of high energy photons is small and has no practical application that I know of, and physics usually derives from engineering, not the other way around. 

[alancalverd]

How can it be used to explain other closely related phenomena, such as reflection, total internal reflection, absorption, and polarization?

It can't, but momentum transfer is the basis of the Mossbauer effect.

[hamdani yusuf (OP)]

You will have to ask them.

Lack of a proper education, perhaps? Failure to respect true genius? Political Correctness (Peterhouse was one of the last colleges to admit women)? Maxwell was considered a bit odd, socially, at the time.

More likely: Few people study both classical and x-ray optics.The refraction of high energy photons is small and has no practical application that I know of, and physics usually derives from engineering, not the other way around. 

If the theory were accurate enough to be used as first principle in engineering, it would be widely used, because rejecting it would bring competitive disadvantages.

[hamdani yusuf (OP)]

How can it be used to explain other closely related phenomena, such as reflection, total internal reflection, absorption, and polarization?

It can't, but momentum transfer is the basis of the Mossbauer effect.

At least we know that there's still a gap in understanding of refraction.

We do not believe any group of men adequate enough or wise enough to operate without scrutiny or without criticism. We know that the only way to avoid error is to detect it, that the only way to detect it is to be free to enquire. We know that the wages of secrecy are corruption. We know that in secrecy error, undetected, will flourish and subvert.
J. Robert Oppenheimer

[Origin]

At least we know that there's still a gap in understanding of refraction.

A more accurate statement is:
At least we I know that there's still a gap in [my] understanding of refraction.

[Bored chemist]

If the theory were accurate enough to be used as first principle in engineering, it would be widely used, because rejecting it would bring competitive disadvantages.

It is accurate and they do use it.
Did you think you had a point?

[alancalverd]

Refraction is sufficiently well  understood by those who need to, that we have microscopes, telescopes, spectacles, and in my case replacement eye lenses and varifocal specs, that work remarkably well. Anglers understand it well enough to be able to drop their bait in front of the fish, and aviators know about the apparent displacement of long-wave and medium-wave radio beacons. The guys who design camera lenses have been using chromatic dispersion data for as long as compound lenses existed - the popular tessar formulation being at least 120 years old.

The adverse refraction of x-rays is known to crystallographers.  IIRC it isn't of much significance in industrial crystallography where you are usually comparing actual with standard diffraction patterns, but bent-crystal monochromators used for some research applications probably need a refraction correction. It's all a very old memory!