This project models the scattering of particles from a target that repels them. An example of this is the scattering of alpha particles (helium nuclei) from a heavy nucleus such as gold. This experiment, first done by Rutherford, provided important evidence that the positive charge in an atom is concentrated in a small place.
For detailed instructions on using the program, take a look at the info window from the Scattering model.
Click on one of the pictures to see a quicktime movie of the model: