Wave/photon Duality. I.

Let's make a list of the phenomena which suggest that light is a wave, and a second list which suggest that light is a particle.


   Behaves like a wave            Behaves like a quantum
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So, wave or particle? Or maybe both? Or maybe neither?

Diffraction is one the phenomena which indicates that light acts like a wave. But doesn't it take more than a single wave to interfere and create a diffraction pattern?

Consider the experiment below: Taylor set up a simple apparatus which took photographs of the diffraction pattern around the shadow of a needle. Nothing fancy there. But he did it in a special way: using light of very, very low intensity. If light travels in quanta of a certain size, then a sufficiently feeble light source should send only one (or zero) photon at a time to any particular area. And if only one photon strikes a particular portion of a photographic plate at a time, how can a diffraction pattern form?

Read the experiment below. Using a bit of additional information:

1 erg = 10^(-7) Joules
assume that all the light has wavelength 550 nm (yellow)
the grain size in photographic emulsion is about 5 microns

try to calculate the number of photons which struck a single photographic grain on the plate each second.