Einstein and the Photoelectric Effect

In order to explain the photoelectric effect, Einstein built on Planck's idea of the quantization of energy, and declared that light was also quantized (he used the term corpuscular).  By this, he meant that light comes in little pieces. Einstein called these pieces photons. 

He explained that the What caused the flow of electrons in the photoelectric effect was photons physically knocking electrons off of he cathode. This was a bold statement that light behaved as a physical entity, not as an amount or type of energy.

To support this radical notion, Einstein explained that the difference between blue light and red light was not the wavelength (since he was saying that light isn't a wave at all). Rather, the difference is that blue light has more momentum (that is, it hits harder). In other words, blue light slams into things like a sledge hammer, while red light hits like a light tapping.

Einstein imagined the electrons as barely hanging on to the metal plate on the cathode and the blue light slamming into them and knocking them off, at which point they could fly across the tube toward the positive plate. Dim light sent fewer electrons across the tube because dim light contained fewer photons. But each blue photon still hit hard enough to knock an electron loose.

Red light, on the other hand, reached the plate with barely a tap -- not nearly enough force to knock the electrons off. Even a VERY bright red light couldn't do it, since bright red light was really just a LOT of little weak taps.

In this way, Einstein took Planck's little constant and gave it life. He says that the constant wan't just a math game, rather it was a description of how the world really works. Light is quantized. It acts as separate little particles, not as a smooth flowing stream.