Double Slit Experiment
The diffraction at a double slit and the characteristic interferences, i.e., the observation of the patterns of superposition, prove the wave properties of the incoming radiation.
Such an experiment is also practicable with electrons. At the beginning the lower slit in the blind is closed so that the electrons can only pass through slit 1. The intensity distribution of the electrons measured by an electron detector positioned at the screen is represented by the red graph (P1). Similarly, the blue graph P2 appears when the upper slit is closed and the electrons can only pass the slit 2.
In case both slits are opened at the same time the electron detector shows a structured intensity distribution (the interference pattern) of electrons, which is represented by the violet graph P12. Although the detector measures electrons one by one and every electron could have either passed the slit 1 or the slit 2, an intensity distribution shows the interference structure. This structure still exists when the electron stream is reduced so much that only one electron at a time makes its way from the electron source through the double slit to the detector. In this case to achieve the interference pattern it is necessary to make a measurement of appropriate long time.