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In 1906, a New Zealand-born British physicist, Ernest Rutherford, did an experiment to test the plum pudding model.

His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles, which have a positive charge, at a very thin gold leaf suspended in a vacuum.

The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.

Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.

After hitting the gold foil, the alpha particles strike the scintillation screen.

A tiny flash of light is emitted when each alpha particle hits the scintillation screen, which can be seen by an observer.

It was thought that the alpha particles would pass straight through the thin foil, or possibly puncture it.

The scientists were very surprised when other things happened:

• most of the alpha particles did pass straight through the foil;
• some alpha particles were deflected by appreciable angles (> 4°) as they passed through the foil;
• a very small number of alpha particles (about 1 in 8000) were deflected by more than 90°, with some alpha particles bouncing straight back off the foil towards the source.

Rutherford was astonished by these results. He is reported to have said:

“It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you!”