Two slit diffraction

Another example of quantum strangeness is provided by a two-slit diffraction experiment. Imagine we have such an apparatus and shine light of a very low intensity at it, so that essentially one photon at a time arrives at the slits, as below.

 

Figure 12.10: A two slit diffraction experiment

What will we find? If we place our measuring apparatus on the screen we will see a diffraction pattern, which is a true signal that a wave has passed through the slits. On the other hand, if we place a photon detector at the slit, then we will detect the presence of a photon if it passes through, which is a particle property, and no diffraction pattern will be observed on the screen. In this way, one can demonstrate both particle and wave properties of light, but not at the same time, simply by changing the type and location of our measuring apparatus. The light ``knows'' when to behave as a wave, and when to behave as a particle, but it never demonstrates both properties simultaneously.