This classic diffraction experiment was conducted in 1911 by Hans Geiger and Ernest Marsden at the suggestion of Ernest Rutherford. Details about the experiment and how to operate the tutorial are provided beneath the applet window.

The tutorial simulates diffraction of alpha particles (helium nuclei containing two positive charges) by a thin foil made of gold metal. Opposite the gold foil is a zinc sulfide screen that emits a flash of light when struck by an alpha particle. To operate the tutorial, use the slider to increase the slit width from a range 0.1 to 9.0 nanometers. As the slit is opened to widths greater than 2 nanometers, some of the alpha particles will collide with the gold nuclei causing them to deflect and deviate from the straight path dictated by the alpha particle source.

Geiger and Marsden expected to find that most of the alpha particles travel straight through the foil with little deviation, with the remainder being deviated by a percent or two. This thinking was based on the theory that positive and negative charges were spread evenly within the atom and that only weak electric forces would be exerted on the alpha particles that were passing through the thin foil at high energy.

What they found, to great surprise, was that while most of the alpha particles passed straight through the foil, a small percentage of them were deflected at very large angles and some were even backscattered. Because alpha particles have about 8000 times the mass of an electron and impacted the foil at very high velocities, it was clear that very strong forces were necessary to deflect and backscatter these particles. Rutherford explained this phenomenon with a revitalized model of the atom in which most of the mass was concentrated into a compact nucleus (holding all of the positive charge), with electrons occupying the bulk of the atom's space and orbiting the nucleus at a distance. With the atom being composed largely of empty space, it was then very easy to construct a scenario where most of the alpha particles passed through the foil, and only the ones that encountered a direct collision with a gold nucleus were deflected or scattered backwards.