Einstein’s Three Great Discoveries. He explained how the motion of atoms could be seen. He proved that light, which was not well understood, could be understood as being little “bullets” of light.
For hundreds of years, biologists had seen little pieces of dust wander around in their field of view in their microscopes.
This was not understood.
Albert Einstein thought about what could be causing it. In the first of three great discoveries in 1905, he realized that it was probably caused by atoms randomly bouncing into the piece of dust.
In reality, the little red atoms are too small too see. But you can see their effect on the speck of dust.