## Do all photons move at the 'speed of light'?

Most people know about the index of refraction, and the corresponding speed v=c/n.

Hecht, in his ubiquitous textbook "Optics", comments that this description is "from a macroscopic point of view" and "misleadingly superficial." (pg 86)

He clarifies on page 92, "photons only exist at a speed c."

So there is the short answer to your question. Yes, all photons move at c, the speed of light.

But that leaves us wondering what is going on when light moves inside of a medium like water or glass, and why we use the equation v=c/n.

When a photon encounters a medium, the electric field in the photon interacts with many electrons in the material. Those electrons (in the classical description), oscillate at frequency $\omega$, as determined by the color of the incoming light. There is a crucial phase difference between the electrons and the photon. The electrons create wavelets in the medium, and each acts as a source. The resulting wavelets overlap and travel at the phase velocity, in the direction determined by snell's law. The phase velocity is what travels at v=c/n.

If the incoming light contains more than one frequency, then each frequency component travels at its own phase velocity, and the wave packet that is the combination of all frequencies travels at the group velocity.

Both the group velocity and the phase velocity contain no information and can travel faster than c!

Of course, this is all explained in the classical picture. But the results are in agreement with the theory of quantum electrodynamics - the most "real" description we have of how light behaves - even if the method of getting there differs.

The Speed of Light, that is speed of Photon particles remains same regardless of distance in Vacuum(299,792,458 m/s). Since, there is no resistance of any king in space, the speed remains unchanged.

However, there is something called refractive index, that is when the photons travel through any object(transparent, translucent, or opaque), the speed, along with angle of travel changes. The formula for its calculation is:

v= c/n

n= refractive index of medium

c= speed of light in vacuum

v= speed of light (medium)/ phase velocity

As we know, Water is denser than Vacuum (Obviously duh!), and its refractive index is 1.3333. Thus, speed of light will be 1.3333 times slower in water, when compared to vacuum.

Reference:

Refractive index - Wikipedia

This question arises because of the anomalous concept of the duality of existence of light i.e. that of a wave and that of a particle.

Because light is a form of energy, it is inferred that light is a particle. They call this particle a photon. But for a particle with a mass value, it cannot travel at light speed because its relativistic mass must be infinity. To skirt this anomaly it is conceive that a photon is a virtual mass with a zero rest mass value. Now, if a photon is a virtual particle can it really move across space?

To take a close look at this, we must first consider that there is no such thing as a completely empty space. Space is not independent of its contents. a ‘vacuum' therefore is not totally empty. It can be devoid of physical mass but it still contains something. This something is called Zero Point Energy or light matrix. Since it is energy, then it should also have equivalent virtual mass.

Now, if ‘empty space' contains virtual ZPE masses, can the photon traverse through ‘empty space'? The answer would be negative.

What could happen is that the electromagnetic energy of the photon is transferred to the adjoining ZPE particle, converting that ZPE particle into a photon while the original photon drops into ZPE particle. This transfer of energy and conversion of virtual particles keeps going on from one virtual mass to another in a billiard ball fashion until at the end of the line the last arising photon is absorbed by the recording instrument. Thus there is an energy wave traversing across a string of virtual ZPE particles but the proton does not move.

Yes, however, tests have shown that photons can form bonds with each other, and these particles can move slower than the speed of light. Single photons move at the speed of light, photons bonded together won't.

sources:

Article explaining original paper:Bonded photons represent a new form of matter

For all intents and purposes, photons are light. Therefore, it doesn't matter what the actual speed they're travelling at, that's the speed of light.

This speed can vary depending on the medium. The speed at which light travels in a vacuum is greater than the speed at which it travels through water, glass, or air.

So yes, all photons travel at the speed of light for the given medium. Change the medium, change the speed.

Yes and it has to be the case.

No massless object can travel slower than c.

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