In a vacuum, the speed of light is absolutely the same, and each medium has a separate delay. But why do the light frequencies in a prism have a different speed due to dispersion? The medium is exactly the same, isn’t it? Or should I use that greater refraction of, say, blue vs. see red light as an accelerated motion comparable to the ‘refringence’ a comet gets when it is pushed away by a larger planet?
Answer
Hi Loes,
The answer isn’t that simple, but I’ll try to explain it without making it too difficult. For this I have to take a few steps back: we first have to ask ourselves why certain materials are transparent to light. When we understand that we can go further into the origin of dispersion.
In the vacuum, the light does not collide with other particles (e.g. atoms) so that it can travel unbridled. However, this is totally impossible in many media: for example, light cannot propagate through a metal because it is immediately absorbed (and partially reflected). This absorption is due to the relatively dense stacking of atoms (or multi-atom molecules) in a liquid or material. Each atom is made up of a very compact nucleus with a positive electric charge that is surrounded by negatively charged electrons. One can intuitively accept that light (an electromagnetic wave) easily collides with electrically charged particles and that light cannot therefore simply propagate through a dense stack of atoms. It is rather surprising that certain media are transparent to light. Transparent media such as glass and water thus exhibit counter-intuitive behavior. In order to explain this transparency (and dispersion), in principle a quantum mechanical description of the electrons in a material or liquid.
Quantum mechanics is a branch of physics that allows to describe the behavior of small particles, eg. electrons and photons (= light particles). This does not work at all with the classical laws of physics, such as eg. Newton’s laws and Maxwell’s equations (Coulomb’s law, Lorentz’s law, …). However, it would lead us too far to fully explain quantum mechanics in this answer. Therefore, I will summarize the main conclusions of quantum mechanics for this question:
- A first observation is that light consists of photons. These are particles whose energy corresponds to a wavelength (the color in the case of visible light). Each color corresponds to a different amount of energy per photon. (The shorter the wavelength, the higher the energy per photon.)
- A second An important result is that electrons in a medium arrange themselves in so-called energy bands. An electron cannot take on any energy in a material or liquid. Certain intervals are allowed while other intervals are not accessible to electrons. Some of the low energy bands are filled by electrons while higher bands remain empty. In the case of a metal, there is always a band that is only partially filled.
When a photon is absorbed in a material, an electron is “excited”: the electron absorbs the energy of the photon and the photon disappears. This means that the electron moves to a higher band. (Metals are an exception to this because an electron within the same band can assume a higher energy.) If a photon just has an energy that cannot be absorbed by the electron because of the tire structure, then this photon can propagate through the medium. This explains why certain media are transparent (to certain wavelengths of light). It can further be shown that the speed at which a photon propagates through a medium depends on (among other things) the energy of the photon. The closer this energy is to the energy of a photon that can be completely absorbed, the more the photon is slowed down. This phenomenon explains dispersion.
Kind regards,
Show Verstraelen
Answered by
Prof. dr. dr. ir. Toon Verstraelen
Molecular Modeling
http://www.ugent.be
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