# Speed of light confusion

#### GENIERE

It seems many posters on these forums as well as others make a distinction between “light” and a “photon” when discussing the speed of light. Most will agree that an individual proton propagates at C whether passing through air, lead, glass or a vacuum. If one defines light as a packet of photons, or a shower of photons, it allows the possibility of saying, “the speed of light is medium dependant” while knowing that an individual photon propagates at exactly C.

An example of the confusion caused by the distinction can be found in refraction tables developed by persons of great authority wherein they assign to light, a velocity dependant on the properties of the medium. The title of the tables usually refer to the “speed of light”. They are either ignorant or, more likely, distinguish between light and a photon.

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#### Chagur

Huh?

Most will agree that an individual proton propagates at C
whether passing through air, lead, glass or a vacuum.

Not me!

#### Integral

Staff Emeritus
Gold Member
Originally posted by Chagur
Huh?

Most will agree that an individual proton propagates at C
whether passing through air, lead, glass or a vacuum.

Not me!
Gamma Ray photons will penatatate a significant thickness of lead.

#### Chagur

Okay, which are you discussing? Protons or photons?

Besides, penetrating a 'significant thickness of lead' is not the same as 'passing through' it.

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#### neutroncount

Originally posted by Chagur
Huh?

Most will agree that an individual proton propagates at C
whether passing through air, lead, glass or a vacuum.

Not me!
Yeah, he meant photons Chagur

#### Integral

Staff Emeritus
Gold Member
Originally posted by Chagur
Okay, which are you discussing? Protons or photons?

Besides, penetrating a 'significant thickness of lead' is not the same as 'passing through' it.
It certianly is, if you have less then that significant thickness!

#### Chagur

I really don't need the attempt at word play, Integral.

Penetrate =/= pass through ... Check a dictionary.

#### Integral

Staff Emeritus
Gold Member
I fail to see the difference. This is physics not english.

Edit:
How do I look up pass through in a dictionay? If a gamma ray is able to penatrate x meters of lead ( if you wish, do some research and replace x with an actual number) and you have less then x meters of lead surrounding your gamma source there will be many gamma rays which pass through the lead. Why is this a word game?

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#### Chagur

The original post, to which I replied, referred to 'Light', not 'gamma radiation'

Sorry for not immediately bringing it to your attention and
avoiding the futile word play you wish to engage in.

And please don't retort with: 'But they're just different
energy levels of the same phenomena.'

#### Integral

Staff Emeritus
Gold Member
Originally posted by Chagur
Huh?

Most will agree that an individual proton propagates at C
whether passing through air, lead, glass or a vacuum.

Not me!
Where does this post refer to the visible portion of the spectrum?

edit: I must admit that I completed missed the replacement of Proton for photon. Since the entire thread is about photons, I simply read it as was intended, not as written. Of course, no proton has ever achieved the speed of light, so it makes no sense if it is read literally.

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#### Bartholomew

Chagur, are you serious?

#### Chagur

And that is what I questioned, I thought, Integral.

Not being that up on Physics, I was willing to grant
that protons, like photons, could pass through air,
glass, and a vacuum; but the lead bit didn't seem
right and I questioned it.

#### Chagur

Yes, Bartholomew.

#### russ_watters

Mentor
It was a typo, Chagur. He meant "photon."

So to clarify: a photon always travels at C. Refraction is often oversimplified to imply that it doesn't. This is especilly true in high school physics classes.

#### Chagur

Okay, russ_watters.

Thanks.

#### GENIERE

OOPS! I guess I added confusion to the confusion. Didn't mean pRoton, nor to start a little spat.

#### Chi Meson

Homework Helper
Originally posted by russ_watters

So to clarify: a photon always travels at C. Refraction is often oversimplified to imply that it doesn't. This is especilly true in high school physics classes.
That would be SOME high school physics classes! Many textbooks (Paul Hewitt's especially) try to make it clear that light waves/photons always travel at c (disregarding the latest theories of variances).

The net propagation of light is slowed by the photons interactions with the atoms and molecules of the substance it is passing through. But while the photon is travelling from atom to atom, it is propagating at c.

#### Bartholomew

Chagur, "penetrate a certain distance" of a material is the same as "penetrate through" the material, if the material isn't as thick as the distance. It doesn't matter whether the thing doing the penetrating is a photon, a proton, or a bullet.

You accept that light can pass through glass. It certainly wouldn't pass all the way through 10 miles of smoked glass. It would penetrate only to a certain distance. But if you reduced the thickness of the glass, eventually light would start coming through in significant amounts--it would start penetrating all the way through.

The same thing is true for lead, on a smaller scale. Instead of feet of glass that the photon can pass through, you have fractions of a millimeter of lead. The light will penetrate that far, and if you can hammer out the lead thinner than that fraction of a millimeter, the lead will be transparent.

#### nautica

"But if you reduced the thickness of the glass, eventually light would start coming through in significant amounts--it would start penetrating all the way through."

When you say "eventually", are you referencing time or thickness??? I first read it as time, but assume thickness. Right???

Nautica

#### S = k log w

Are the photons electron free?

#### russ_watters

Mentor
Originally posted by S = k log w
Are the photons electron free?

Huh?

#### yogi

"All these years of conscious brooding about the photon have brought me no closer to the truth - nowdays every Tom, Dick and Harry thinks he knows the answer, but he is mistaken."

Albert Einstein in a letter to his friend Besso near they end of his life.

#### Integral

Staff Emeritus
Gold Member
Originally posted by yogi
"All these years of conscious brooding about the photon have brought me no closer to the truth - nowdays every Tom, Dick and Harry thinks he knows the answer, but he is mistaken."

Albert Einstein in a letter to his friend Besso near they end of his life.
There was an interesting claim made in the PBS Nova, "Elegant Universe"

From about 1920 on Einstein did not keep up with the mainstream of Physics. He did not keep up with or understand the QM model of the Atom which he initiated with his Noble Prize winning paper on the Photoelectric effect. He concentrated on his eforts of Unification without the essential knowledge of QM. His efforts were doomed without that information.

So any quote from Einstein about understanding QM is pretty meaninless. He choose to ignore it.

#### yogi

Integral - the fact that Einstein did not agree with some interpretations of QM does not mean his views should be discarded - or that they were unfounded...Einstein, unlike you and a lot of the others who post on these boards, continually re-thought things from a conceptual perspective - his own contributions as well as those of others - and if you knew more about his life you would not say that he didn't keep up - he spent many hours thinking through experiments that would invalidate the Copenheagen interpretation - "In matters of science the authority of a thousand is not worth the humble reasoning of a single individual" Einstein always worked alone "I am a horse for a single harness - not made for tandem work" His great contributions were done solo - and he continued in that style - If these boards are only to regurgitate the stuff from text that have been copied from each other - they are a waste of time - Ever thought you were really sure of something and found out you were wrong??

#### FZ+

The same thing is true for lead, on a smaller scale. Instead of feet of glass that the photon can pass through, you have fractions of a millimeter of lead. The light will penetrate that far, and if you can hammer out the lead thinner than that fraction of a millimeter, the lead will be transparent.
I'm guessing we are using a useful version of pass through where we do not insist that we have an infinite thickness of medium where nothing can pass through.

Anyways, a finite amount of lead can not completely block off light. Fire enough photons at it and eventually one will be lucky enough to get through.

Integral - the fact that Einstein did not agree with some interpretations of QM does not mean his views should be discarded - or that they were unfounded...Einstein, unlike you and a lot of the others who post on these boards, continually re-thought things from a conceptual perspective - his own contributions as well as those of others - and if you knew more about his life you would not say that he didn't keep up - he spent many hours thinking through experiments that would invalidate the Copenheagen interpretation
Indeed. The most powerful one was the so-called EPR paradox. It was conducted. Einstein was shown to be wrong.

Ah well. And that's the end of that.

Photons and wave theory are two sides of the same coin. Physics usually mean "em radiation" when they say light.

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