# Reflection: instantaneous or not?

1. Jan 30, 2012

### elegysix

when light encounters a mirror, is it instantly reflected, or is it absorbed very briefly and then emitted?

2. Jan 30, 2012

### DaveC426913

Presumably, you don't care about the thickness of glass in most mirrors. Ideally, you're talking about a mirror whose reflective surface is at the front, not the back.

Last edited: Jan 30, 2012
3. Jan 30, 2012

### Bobbywhy

As DaveC426913 says, if it is a "first surface" mirror then there is no delay whatsoever.

The ordinary bathroom mirror is a "second surface" mirror. A pulse of light first passes through the thickness of the glass, gets reflected at the backside silvering, then travels out through the glass again. This round trip through glass would "delay" the pulse because the light actually travels slightly slower through glass than it does through air.

4. Jan 30, 2012

### morrobay

'Gets reflected at the backside silvering'

I interpret the question in the mechanism in this part of the process.
Is the light absorbed and re emitted in the backside silvering ?

5. Jan 31, 2012

### edguy99

I believe in the case of a mirror, the photon acts as a wave and is simply reflected. Very different from say a wall painted green. There the photon is absorbed, an electron is bumped out of place, another electron falls in its place and a photon is shot back out, often in a different random direction. The green paint has the property that most of the photons coming out are a specific wavelength that looks green to us due to the molecular structure of the paint.

6. Jan 31, 2012

### morrobay

7. Jan 31, 2012

### clem

The simple answer is: It is instantly reflected, because the interference at the surface between the incident and reflected wave takes place at the same time for each.

8. Jan 31, 2012

### morrobay

So you are defining the process where the alternating electric field vector of the incident
light wave that causes the free conduction electrons in the silver mirror to oscillate , and
then radiate the reflected wave , instantaneous ?

9. Feb 1, 2012

### clem

It has nothing to do with the mechanism,
"because the interference at the surface between the incident and reflected wave [must take] place at the same time for each.

10. Feb 1, 2012

### Rap

When viewed as a classical electromagnetic wave reflecting from a surface, the delay is zero. Looking at it in more detail, you can think of it as a classical wave striking a bunch of free electrons and there is no "surface". The electrons are accelerated and decelerated by the field and so they emit an electromagnetic wave of their own, and this process takes time. As you go further into the body of the reflector, the wave is shielded by these electrons and after a certain small distance, the impinging wave is practically absent. The wave from an oscillating electron will interfere with the waves from other electrons, and only very near an angle of reflection equal to the angle of incidence will their waves interfere constructively, as long as the dimensions of the reflector are large compared to the wavelength of the incoming radiation. The full answer is given in quantum electrodynamics, where an incoming photon is absorbed by an electron, which then emits a photon in a more or less random direction. The incoming photon rarely gets very far into the material, but the absorption and emission process take a small amount of time. Again, only very near an angle of reflection equal to the angle of incidence will the wave function of the photons interfere constructively, as long as the dimensions of the reflector are large compared to the wavelength of the incoming radiation.

11. Feb 1, 2012

### clem

If the wavelength is long compared to the molecular spacing (as for light), a reflected photon does not interact with only a single electron.