# Reflection and refraction

curioso
I am not a physicist but I do have a non-mathematical science background and I am a serious physics appreciator. I am new to this site and have a pressing question I would like to ask a physicist.

High school and even college level physics texts desribe the reflection and refraction of light as bouncing and bending phenomena respectively. But how can this be? At the quantum level do photons really "bounce" off of electron clouds -- and do the paths of individual photons really "bend" as they move from one transparent medium into another. I find this a little hard to swallow.

Are these phenomena not really absorption and re-emission events?

Thanks.

## Answers and Replies

Gold Member
It's hard to describe the exact mechanisms but in general it's just generic wave physics. Waves, when incident upon various boundaries, exihibit interference, reflection, refraction, diffraction and probably some other -ractiony things.

Part of this has to do with the interaction of the boundary with the wave. For example, if a sound wave hits a wall, the wall actually will vibrate. If an electromagnetic wave hits a conductive surface, it will induce currents on and inside the surface. These effects are sources for secondary waves. The combination of the incident wave and these secondary waves gives rise to the total wave that you observe. So if you consider that incident waves create sources on and inside objects, you can get a better sense for how these waves can bend around corners; because there are waves being created throughout the object. Cancellation of the secondary and incident fields arise due to constructive and destructive interference. This is how reflection occurs. The incident wave constructively interferes with the secondary wave in front of the boundary. But inside and behind the boundary the two destructively interfere. The end result is that there are no waves inside or behind the boundary but there is a reflected wave in front of it.

Photons will exhibit the same basic wave phenomenon. We can observe in experiments even single photons interfering with themselves or diffracting through slits and so forth. However, modern quantum theories do not consider the photon to have a physical path. At best, we talk about being certain of a photon being sourced at point A and being absorbed (say by measurement of a detector) at point B. How the photon behaves between its creation and annihilation is undefined. In fact, if we were to have a series of detectors that observed the path of the photon from A to B, we would destroy the quantum effects. For example, as I mentioned before, we can see single photon interference in a double slit experiment. However, if we were to detect which slit the photon passed through, we destroy the interference (but we still get diffraction).

curioso
Dr, Mr or Ms Born2bwire...

Thanks for this. It gives me lots to ponder!

Curioso