Photoelectric effect

See this video

Summary:
Plank assumed in explaining Plank's law that when light energy is transmitted, it doesn't behave lake plain water poured from a glass into another, but like if we transfer the water with a spoon, whose size depends on the "color of the water" (frequency of the light).

But Planks didn't give an explanation, Einstein did: the reason is that light energy is not water but ice cubes.
This way, he could explain the photoelectric effect: the electron needs to be hit by a big enough ice cube. Bigger cubes extract electrons transferring them more kinetic energy.

Einstein couldn't explain why many small cubes (augmenting the intensity of red light) could not trigger an electron. But currently we know that it is possible, but we need an insane amount of red cubes, due to the fact that it is unlikely that two cubes (photons) hit the electron simultaneously.

A useful device: electroscope