Rutherford performed an experiment, now called the Rutherford model, where a beam of alpha particles was shot to a piece of gold foil. This gold foil was surrounded by a flourescent screen. When the alpha particles hit the screen, the screen would light up. When observing this, you can see that the beam did not go straight through the foil. This caused Rutherford to believe that an atom must have some kind of nucleus and when it is hit with the particles, they are deflected.