Light, Photon Waves, and Electromagnetic Waves

Most recent answer: 04/26/2016

Q:
I've recently read about wave-particle duality. What I understand is that the fundamental building blocks of matter act as both particles and waves. Before observation, they exist as a wave function which gives the probability of finding the particle at a particular place. Light also exhibits this property. We say that light is electromagnetic wave. We also say that light is a photon. But instead of probability wave, light is said to be an electromagnetic wave. So basically I'm getting confused with probability and electromagnetic waves in case of light. I guess photon has two meanings- one for EM light and other as a fundamental particle exhibiting dual nature? What does EM force have to do with light?
- Malay (age 15)
India
A:

Yes, the role of these different descriptions of light can get confusing. The underlying exact description (so far as we now now) is that there's a quantum wave, a photon wave. It shouldn't be thought of as a collection of photons but rather as a wave with possibilities for a range of different numbers of photons, in the event that it happens to run into something like a photon-counting device. It also has a range of possible electromagnetic fields, if it happens to run into a field-measuring device. One peculiarity is that if the quantum wave has some exact number of photons, rather than a range, then the average electric field (also magnetic field) on repeated measurements of waves like that would be zero. 

So the classical field description is an approximate description that works very well when the underlying field has lots of photons and the right sort of spread of possible photon numbers to give classical field behavior. Ordinary light sources usually are like that. You can also make beams with very different properties, well-defined numbers of photons and nothing like well-defined classical EM fields.

Mike W.


(published on 04/26/2016)