It is sometimes claimed that light is slowed on its passage through a block of media by being absorbed and re-emitted by the atoms, only traveling at full speed through the vacuum between atoms. This explanation is incorrect and runs into problems if you try to use it to explain the details of refraction beyond the simple slowing of the signal.
Classically, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.
The full quantum-mechanical explanation is essentially the same, but has to cope with the discrete particle nature (see Photons in matter): The E-field creates phonons in the media, and the photons mix with the phonons. The resulting mixture, called a polariton, travels with a speed different from light.
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Light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. For example, photons suffer so many collisions on the way from the core of the sun that radiant energy can take years to reach the surface; however, once in open space, a photon only takes 8.3 minutes to reach Earth. 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 the 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 (quasi-particles such as phonons and excitons) to form a polariton; this polariton has a nonzero effective mass, which means that it cannot travel at c. Light of different frequencies may travel through matter at different speeds; this is called dispersion. The polariton propagation speed v equals its group velocity, which is the derivative of the energy with respect to momentum.
I extend my question. Does hf work in the medium?pervect said:Zapperz is the best person to ask, though I think he's written it down in the FAQ and is a bit tired of repeating himself. I suppose I ought to go re-read the FAQ myself.
pervect said:OK, I found the FAQ entry, https://www.physicsforums.com/showpost.php?p=899393&postcount=4
Without beingt interested in the mechanism which slows down the propagation of the e.m. wave, we could consider that a Doppler effect could take place, the equation which accounts for it relating the frequencies detected by two observers relative to whom the wave propagates with u and u' respectively the relative velocity of the two observers being V, which could exceed u and u'. Under such conditions f and f' (the frequencies detected by the two observers) are related by a Doppler shift formulas simillar to those we find in the case of a mechanical wave related by a Doppler factor D which depends on u(u'), c (because of the clock sybchronization) and V. Do the equations hf=hf'D work?
Consider please that what I say is a question and not a statement.
Thanks. You understood exactly my question. I think that only in your reply should be extended as: The Doppler shift factor depends in that case on the relative velocity of the two involved observers, on the velocity at which the wave propagates relative to the two observers and on c, due to the fact that in both reference frames clock synchronization is performed. From that point of view the Doppler shift formula should have, in the case of the longitudinal case, the same algebraic structure as the Doppler shift formula in the case of an acoustic wave has. My problem is that if in the case of the e.m. wave propagating through the medium, the frequencies f and f' detected by the two observers are related bypervect said:I'm not quite sure what you are asking. In SR in a vacuum, the doppler shift factor will depend only on the velocity of the two observers relative to each other - in this case, the doppler shift factor will depend on the speed of each observer relative to the medium.
A photon is a fundamental particle of light that carries energy and momentum. It is associated with an electromagnetic wave because electromagnetic waves are composed of oscillating electric and magnetic fields, and photons are the building blocks of these fields.
Photons travel at the speed of light, which is approximately 299,792,458 meters per second in a vacuum. This speed is a fundamental constant in the universe and is a key aspect of the relationship between photons and electromagnetic waves.
No, photons cannot be seen with the naked eye because they have no mass and do not interact directly with our eyes. However, they can be detected using specialized equipment, such as a photodetector, which converts the photons into electrical signals that can be interpreted by the brain as light.
The energy of a photon is directly proportional to its wavelength. This means that photons with shorter wavelengths, such as gamma rays, have higher energy than photons with longer wavelengths, such as radio waves.
Photons make up the electromagnetic spectrum, which includes all types of electromagnetic radiation, from radio waves to gamma rays. The different types of photons have different wavelengths and energies, and the entire spectrum is crucial for understanding the behavior and properties of light.