After Einstein announced the photoelectric effect, the British physicist Ernest Rutherford performed his new-famous gold-foil experiment. This 1911 experiment showed that the atom was mostly empty space, with most of its mass packed into the central region, the nucleus. Rutherford and his team directed a beam of positively charged particles from a radioactive source through a very thin gold leaf. They measured the angles at which the particles were deflected from their straight-line path as they emerged from the gold foil. Rutherford determined the angles of particle deflection by noting spots of lights on a zinc sulfide screen around the gold leaf. Most particles continued, as expected, in a basically straight-line path after going through the foil. But, some particles were scattered back along their incident paths. As Rutherford later put it, ” It was as though you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Rutherford reasoned that the particles that were only slightly deflected traveled through mostly empty space in the gold foil, while any particle deflected through a large angle must have experienced a very strong force by coming close to a concentration of positive charge at the center of an atom. Rutherford had discovered the atomic nucleus.