In 1909, the prevailing theory of the atom's structure was that atoms were mushy, semi-
permeable balls, with bits of charge strewn around them. This theory worked just fine
for most experiments about the physical world.
Physics, however, is not only interested in how the world appears to
operate, but how it actually works. And so in 1909 a man named
Ernest Rutherford set up an experiment to test the validity of the prevailing
theory. In doing so he established a way that for the first time physicists could "look into"
tiny particles they couldn't see with microscopes.
In Rutherford's experiment, a radioactive source shot a stream of alpha particles at
a sheet of very thin gold foil which stood in front of a screen. The alpha
particles would make little flashes of light where they hit the
The alpha particles were expected to pass right through the very thin gold foil and make
their marks in a small cluster on the screen.