Reflection Instant? Atomic Level Causes Explored

[Aaronazi]

TL;DR Summary

In the reflection of light, does the striking of a body by the incident ray happen at the exact same time that a reflected ray starts to reflect

What causes reflection on the atomic level? I believe that the interactions of light and the electrons of a body take at least very very small amount of time. The time is probably negligible but does that time even exist or does the reflection instantly happen.

 

[Dale]

I am not sure how this could be measured.

 

[DaveC426913]

I am not sure how this could be measured.

Set up an experiment where one unreflected (or minimally reflected) beam is calibrated with one that is reflected tens of thousands of times.

 

[Baluncore]

What causes reflection on the atomic level? I believe that the interactions of light and the electrons of a body take at least very very small amount of time. The time is probably negligible but does that time even exist or does the reflection instantly happen.

Reflection seems to involve two cross products, that turn the wavefront by generating a counter field in less than the period of the radiation. It is hard to see how there could be much phase shift in such a phase referenced process, without a significant loss of reflectivity.

Any delay in reflection is less than the time it takes light to cross the ionic radius of an atom.
It is difficult to measure the position of atoms that form the mirror, so it will likewise be difficult to estimate any time delay.

 

[Andy Resnick]

Summary:: In the reflection of light, does the striking of a body by the incident ray happen at the exact same time that a reflected ray starts to reflect

What causes reflection on the atomic level? I believe that the interactions of light and the electrons of a body take at least very very small amount of time. The time is probably negligible but does that time even exist or does the reflection instantly happen.

There has been some theory developed for this in the context of ultrashort pulses; there are differences with steady-state results but I can't remember any specifics. The underlying reason is the time dependence of the induced polarization within matter, the material response is not instantaneous.

 

[Baluncore]

It is hard to see how there could be much phase shift in such a phase referenced process, without a significant loss of reflectivity.

Maybe the reduction in reflectivity at progressively shorter wavelengths (and periods) could be used to infer a delay time phase error in the reflection process.

 

[hutchphd]

The most precise ways to measure short time delays for a square edge involve the phase delay for harmonic waves. For instance a phase delay linear in frequency generates a fixed time delay. (Phase linear amp!)
I helped design a medical instrument that measured fluorescent lifetime (which indicated a concentration-dependent decay mechanism) by exciting the fluor with 60MHz LED light and looking at the phase shift of the emitted fluorescence.
So when possible this is much easier than lots of go-=fast optics and pulses.

 

[tech99]

When an EM wave is reflected, electrons in the surface move in sympathy with the incoming electric field, and in so doing they undergo acceleration. When the field is at its maximum, the accelerating force is greatest. The electrons emit radiation in-phase with the acceleration, which we see as reflected energy. If the electrons had noticeable mass, their motion would be delayed relative to the incoming E-field, but their mass is extremely small so we do not see a delay. If the electrons had noticeable mass, their motion would be lagging the incoming E-field and the surface would radiate a lagging wave, so would be inductive. At wavelengths in the ultra violet region, with metals, we start to see surface resonances which are related to the mass of electrons.

 

[Orthoceras]

If reflection caused a phase shift (other than the 180° phase shift at the air-glass boundary), it would be visible in Newton's rings. The central patch would not be black.