Now lets turn down the intensity of the light source. For very low intensity I find that my detector
collectsone photon at a time. It never collects half a photon. (With the right detector, I could
again verify that for each photon, the Photoelectric effect is seenand thatE=hν.) So the waves
that are diffracting are somehow made up of photons. With alow enough intensity, I can assure
thatonly one photonis present in the apparatus at any time. I can operate my detectorand
collect data over a long time, summing up the number of photons detected as a function of position.
What will I get for the distribution? I get exactly the same distribution as before with maxima and
minima. No matter how low the intensity, (1 particle/ minute!)we still see diffraction. We never
detect a fraction of an electron or a photon,only integer numbers.
How does a single photon interfere with itself? It must somehow travel through both slits.
Lets turn toelectron diffractionfor a minute. In our thought experiment we again have two slits.