Caption: "Results of a double-slit experiment performed by Akira Tonomura (1942--2012) showing the build-up of a diffraction pattern from single electrons. The counts of electrons are 10 (a), 200 (b), 6000 (c), 40000 (d), 140000 (e)." (Slightly edited.)

Features:

1. Electrons like photons are subject to the wave-particle duality of quantum mechanics---all microscopic particles are.

2. The diagram illustrates the double-slit experiment with electrons and the wave-particle duality.

3. Each electron is in a continuum superposition of positions described by a wave function---whose common symbol is the capital Greek Psi = Ψ.

   The wave function of each electron passes through both of two slits on a screen. Which means each electron passes through both of two slits on a screen.

   The intensity of the electron beam in the experiment is so low that there is only one electron at a time passing through the two slits.

4. The wave function then interferes with itself and creates a diffraction pattern which is the probability distribution for an absorption of the electron at one point on an absorption screen.

   When an absorption occurs, we say there has been a wave function collapse to a point.

5. The electrons, via probability distribution (essentially the diffraction pattern) of wave function, gradually create an quasi-continuous absorption diffraction pattern as the number of electrons having passed through the two slits increases.

6. Eventually, the individual electron behavior is washed out.

7. The double-slit experiment can be done with photons with the same results qualitatively speaking---light propagates like a wave and is absorbed like a particle.

8. The photon version actually does NOT prove photons exist---althought the point can be debated---but it is consistent with their existence. Given that photons exist, it does prove that each one goes through both slits and interferes with itself. Final proof that photons are indispensable entities in physics was finally obtained only in the 1970s (e.g., Greenstein & Zajonc, 2005, p. 32--34).

9. Remarkably, the photon version was done first in 1909 by Geoffrey I. Taylor (1886--1975) (see Wikipedia: G.I. Taylor: Biography).