B Help with refraction of light

[Dark85]

TL;DR Summary

Does light undergo infinite deceleration when it passes from one medium to another.

Hey guys I have a question regarding the refraction of light,
When light passes from one medium to another i.e say from air to glass, it's speed changes from 3x10^8 m/s to 2x10^8 m/s. But does this change in speed happen instantaneously? If so is the resulting deceleration infinite?

 

[Ibix]

The reduced speed of light in a medium is due to the interaction between the electromagnetic wave and the electromagnetic fields between the charged particles making up the atoms of the medium. There isn't a sharp start to those fields, so there won't be an instant change from "speed in vacuum" to "speed in medium". It'll be a short distance, but not zero.

 

[Dark85]

The reduced speed of light in a medium is due to the interaction between the electromagnetic wave and the electromagnetic fields between the charged particles making up the atoms of the medium. There isn't a sharp start to those fields, so there won't be an instant change from "speed in vacuum" to "speed in medium". It'll be a short distance, but not zero.

So you are saying there is delay between the instances when light enters rhe medium and the interaction between the electromagnetic wave and field which makes the whole process not instantaneous?

 

[PeroK]

TL;DR Summary: Does light undergo infinite deceleration when it passes from one medium to another.

Hey guys I have a question regarding the refraction of light,
When light passes from one medium to another i.e say from air to glass, it's speed changes from 3x10^8 m/s to 2x10^8 m/s. But does this change in speed happen instantaneously? If so is the resulting deceleration infinite?

Light obeys Maxwell's equations, not Newton's laws of motion. The concept of "deceleration" would normally apply to a particle under some force. That's not the right model for light. Instead, light is oscillating electric and magnetic fields. No physical object decelerates when light goes from air to glass - or vice versa. Instead, the fields have different parameters in the two media.

Normally, the interface between air and glass is modelled as a perfect, plane barrier. In fact, the case of reflection of light from a glass surface is more pointed: the light hits the glass/mirror at the speed of light in one direction and reflects instantaneously at the speed of light in the opposite direction.

That is the classical model of light. Athough, we can see that it must be an idealisation, as the glass is composed of atoms and molecules and is not a perfect, plane barrier.

To understand reflection and refraction at a deeper level requires the quantum theory of light (QED - Quantum Electrodynamics). Richard Feynman has a book and some lecture notes on this:

https://en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter

In that model, the emission and detection of light becomes probabilistic and even further from Newton's laws of motion. In any case, the concept of deceleration or acceleration as understood in Newtonian mechanics is not part of the theory.

 

[Dark85]

Light obeys Maxwell's equations, not Newton's laws of motion. The concept of "deceleration" would normally apply to a particle under some force. That's not the right model for light. Instead, light is oscillating electric and magnetic fields. No physical object decelerates when light goes from air to glass - or vice versa. Instead, the fields have different parameters in the two media.

Normally, the interface between air and glass is modelled as a perfect, plane barrier. In fact, the case of reflection of light from a glass surface is more pointed: the light hits the glass/mirror at the speed of light in one direction and reflects instantaneously at the speed of light in the opposite direction.

That is the classical model of light. Athough, we can see that it must be an idealisation, as the glass is composed of atoms and molecules and is not a perfect, plane barrier.

To understand reflection and refraction at a deeper level requires the quantum theory of light (QED - Quantum Electrodynamics). Richard Feynman has a book and some lecture notes on this:

https://en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter

In that model, the emission and detection of light becomes probabilistic and even further from Newton's laws of motion. In any case, the concept of deceleration or acceleration as understood in Newtonian mechanics is not part of the theory.

Oh okay, will read Feynman's lectures for sure... thank you so much

 

[sophiecentaur]

So you are saying there is delay between the instances when light enters rhe medium and the interaction between the electromagnetic wave and field which makes the whole process not instantaneous?

You can't really explain / understand this if you choose to think in terms of photons (so called particles) You'll have difficulty thinking about photons slowing up and speeding up again then they pass through a transparent object. That's because photons cannot be regarded as 'being somewhere' at a time; they have to be looked at as being 'everywhere and nowhere in particular'. The wave model explains pretty much all optics without that awkwardness. It's quite ok to pick and choose between the two natures of light as appropriate.

Light crossing a boundary between two different media will just change wavelength as it passes. The actual transition will take place around the interface; there are no 'hard' changes at an interface and the light doesn't gain or lose energy in the process. The frequency always stays the same in the path. The spacing between maxima or 'zero crossings' will be different on either side.

 

[Dale]

So you are saying there is delay between the instances when light enters rhe medium and the interaction between the electromagnetic wave and field which makes the whole process not instantaneous?

I think that what he was saying is that “when light enters the medium” is not a specific instant

 

[Baluncore]

The actual transition will take place around the interface; there are no 'hard' changes at an interface and the light doesn't gain or lose energy in the process.

If there is a change in the refractive index at the interface, then there will be less energy in the onward travelling light, as some energy will be reflected by the impedance mismatch.

 

[sophiecentaur]

If there is a change in the refractive index at the interface, then there will be less energy in the onward travelling light, as some energy will be reflected by the impedance mismatch.

That's true and applies when any wave meets a discontinuity, light, sound, vibrating strings etc etc. And always, the frequency of the wave remains the same. The reason / explanation for that is that there has to be continuity across the interface; the E field (or any other quantity) on one side has to equal to the E field on the other so any difference involves a reflected wave as well as a transmitted wave. Many badly drawn diagrams ignore this so you need to be careful.

 

[Andy Resnick]

TL;DR Summary: Does light undergo infinite deceleration when it passes from one medium to another.

Hey guys I have a question regarding the refraction of light,
When light passes from one medium to another i.e say from air to glass, it's speed changes from 3x10^8 m/s to 2x10^8 m/s. But does this change in speed happen instantaneously? If so is the resulting deceleration infinite?

Applying mechanical analogies to light propagation is generally not advisable. Refraction is associated with changes in light's momentum (wavelength), not speed.
At the material boundary/discontinuity, the near-field behavior can be quite different than far-field behavior, a good example of this is frustrated total internal reflection caused by the evanescent field. Tailoring the dielectric interface to create a desired near-field pattern is currently an active research topic (nanophotonics).