# Photons-Wave/particle duality

I know that photons and electrons and such are said to have a wave particle duality, but what does that mean for a photon? When light strikes an object, are many photons emitted, enough to draw infinitely many rays, is only one emitted, or something in between?

In particular, I'm having trouble with thin film interference:

The two resulting rays are said to constructively interfere, which is confusing to me. The two rays are clearly parallel, but not coinciding, so how do the two interfere at all? I think my problem is that I imagine light to be a single ray, with a linear oscillating magnetic field- what is the proper way to address these rays? Are they photons? Or are they small instances of a wave front? I've heard Huygens' Principle, but in this case we present single rays at the end, so I'm led to believe they really ARE rays, in which case they would be photons, and the interference problem would be a result of the wave/particle duality. The only other thought I've had with regards to the interference is that, as opposed to looking at the rays as one dimensional rays, they could be some kind of representation of a wave 'centered' around that vector, but that doesn't make sense either. I know it's a heavy question, but it's really confusing me.

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If you have a bit of time these lectures by Feynman are the best I know about the subject. – Michael Brown Jul 4 '13 at 2:45
In this problem, rays are representative of the classical EM wave. They describe the path of the wave perpendicular to the wavefront. You shouldn't think of this in terms of photons. The interference comes from the interference of the waves due to the difference in path lengths reflected at $A$ and at $D$ respectively (I'm sure you've looked at addition of two sinusoidal functions before and seen the result of adding them when out of phase?). I can explain further in an answer if this doesn't give you what you want. – Will Jul 4 '13 at 3:06
Maybe I haven't understood what you are asking (Btw replace $D$ with $B$ in my comment above) – Will Jul 4 '13 at 4:34

Anna v's answer is entirely correct, but a simple explanation to merely the thin-film problem is that rays are really waves that have width, so they overlap.

So yes, the ray represents kind of a wave "centered" around that vector. Within a small region of space you can approximate a more realistic wave as propagating only in the forward direction, exactly like a ray.

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So the two rays are simply wavefronts- does this mean that their amplitudes are double for the entire wave front? How does the amplitude of an electromagnetic wave decay over time/space? – user24082 Jul 8 '13 at 0:18
Yes, the two rays are simply wavefronts. I'm not sure what you mean by your question "are their amplitudes double?" Double compared to what? – David Jul 9 '13 at 23:12
Ah sorry, double after being reflected in the thin film example. – user24082 Jul 10 '13 at 6:50

The two resulting rays are said to constructively interfere, which is confusing to me. The two rays are clearly parallel, but not coinciding, so how do the two interfere at all?

You are actually confusing three different physics representations of the behavior of light, each framework self consistent and smoothly transitioning to the other in the region of validity.

There is the classical electromagnetic wave framework, where light is an electromagnetic wave whose amplitude are changing electric and magnetic fields as the wavefront progresses.

Rays are a geometric representation of the way the classical wavefront progresses and transmits its energy and are useful for construction of optical designs.

Photons are the elementary particles that build up the observed macroscopically electromagnetic waves. As discrete particles they are described by their energy content given by the frequency E=h*nu where the frequency nu is also the classical wave frequency of changing electric and magnetic fields.

I think my problem is that I imagine light to be a single ray, with a linear oscillating magnetic field- what is the proper way to address these rays? Are they photons?

Rays are a geometric representation of the wave front, the direction of energy propagation. They are not photons, nor the following

Or are they small instances of a wave front?

They show the direction of the wavefront.

I've heard Huygens' Principle, but in this case we present single rays at the end, so I'm led to believe they really ARE rays, in which case they would be photons, and the interference problem would be a result of the wave/particle duality.

Even though the interference pattern blend smoothly from the framework of individual photons to the classical electromagnetic wave framework , rays are NOT photons. The wave is composed of zillions of photons and rays show the geometric progression of the wave.

The only other thought I've had with regards to the interference is that, as opposed to looking at the rays as one dimensional rays, they could be some kind of representation of a wave 'centered' around that vector, but that doesn't make sense either.

Rays cannot tell you anything about interference, because interference needs a wave equation and rays are not solutions of a wave equation. They are a geometrical representation of the propagation of the energy of the wave front which is composed of zillions of photons.

When light strikes a surface, zillions of photons hit it. Thus the classical representation of electromagnetic waves is the smart way to study photon interference in the lab.

If one is interested in the photon particle/wave duality one has to look at a single photon at the time. In the two slit experiment single photons build up the same interference pattern over time ( a second video). At this microscopic level it is a quantum mechanical probability function that is describing the interference pattern from the elementary particles called photons. It blends with the classical wave description of electromagnetic waves. One can see a similar interference pattern built up by single electrons at a time.

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