Does anybody know if there are instances other than diffraction

that demonstrate the photon as a wave? There are tons of demonstrations showing light as a particle. Heck, even if we send it through one photon at a time, diffraction demonstrates photons as particles!
Where is the photon a wave?


Science Advisor
I am not sure whether you consider the double slit experiment diffraction. However it is the classical demonstration of photons as wave. The effect shows up even when sends one photon at a time. Another variation is that electrons also exhibit the same wave effect.
There are many examples for the wave behavior. Some of these are in fact quite practical, not just merely laboratory setups. For example, take the anti-reflecting coating on a glass. This is done by depositing a very thin layer (one-quarter the wavelength of average visible light in the said medium) of some other transparent substance with a different refractive index. The reflections from the two surfaces interfere destructively eliminating all reflection. (There is some residual reflection always, because the interference can be perfect only for monochromatic light.)
The photon is always detected as a particle. For example, a single photon appears as a dot on a detection screen. It is localized in space-time. Experiments done one photon at a time all indicate that the photon is a particle. No wave properties are evident when we observe a single photon. A single photon is never smeared over the detection screen as a wave would be.
Oftentimes, however, when we repeat the experiment many times and observe many photons, then, we get a statistical distribution of all the dots on the screen due to different photons (particles) arriving at the screen at different times. This distribution of dots has maxima and minima that look like constructive and destructive interference in wave optics.
The point is this: Individual photons are particles without wave properties. Wave properties might be seen only when many, many photons are observed.
The defining experiment was done by Tonomura, et al, Amer. J. Phys. 57, 117-120 (1989)

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