Thread: What causes phase change when light is reflected?

Usually, when light is reflected by an ordinary mirror, it's phase shifts by 1/2 a wavelength. So if 2 beams of light are initially in phase, and one gets reflected, it is exactly opposite in its wave cycle. If the non-reflected beam is at its highest positive state, electrically, the reflected one would be at its lowest negative state, because it is exactly 1/2 a wavelength behind/in front.

So that's really my question: is it because it got advanced 1/2 a wavelength, or because it got delayed 1/2 a wavelength, or neither? (Maybe it simply got converted from positive to negative by the material that reflected it?)

If we used 2 radio waves that had long wavelengths, together with a suitable mirror, and very accurately measured the distances traveled by each, would we find that the reflected wave was arriving later at the detector?

Suprisingly, it is the properties of the reflecting material which is responsible for the half wavelength phase shift. The electrons on the surface of the material respond to the incident light, which causes them to oscilate. When the electrons start to move, they release light, and this phase is opposite to the phase of the light which cause them to move, because they are emitting it in the opposite direction. This is all about action and reaction: for every action there is an equal and opposite reaction.

Do they have to absorb a full photon first before the start emitting?

Originally Posted by kojax

Do they have to absorb a full photon first before the start emitting?

This explanation is based on the wave nature of light, not the particle model. In effect, the electron does not "absorb" a photon and then re-emit it, rather it oscillates continuously in synchronization with the light waves and then releases its own light waves as electrons do when they accelerate.

I think I'm still a little bit confused. You're saying the light wave causes the electron to move, and the movement of the electron causes a new light wave with opposite positive/negative orientation?

So, it really is an instantaneous effect, pretty much? The electron doesn't need to absorb a certain amount of energy first?

I recommend not using the word "absorb", I find that it can be quite misleading. It makes it sound like an electron is a plant soaking up water.

The incident light tranfers energy to the electron in the form of kinectic energy, which causes the electron to accelerate. We know that when a charge accelerates, it gives off electro-magnetic radiation (light). But in order to satisfy conservation of energy, no energy can be created or destroyed in this process, so the electron must release light whose phase is opposite to the incident light so it cancels it out.

So basically, the light is giving energy to the electron, and that energy is re-radiated by the electron as it accelerates, but with a different phase. This process is almost continuous, but there is in fact a very slight delay due to the speed of light.

Here is a website which may help you further with relections and why the phase changes: Reflections