The Photoelectric Effect

Imagine a cathode ray tube, like the one used by Thomson. When a large enough voltage is applied, the glass at the positve end of the tube glows, except for a shadow of the Maltese cross. If you don't understand how that works, go back and look at this page. Generally this experiment is done in a dark room so the glow can be seen more easily.

Now, if the voltage is turned down, eventually the electrons stop flowing and the tube stops glowing. But...if a bright light is shined on the metal plate on the left, the electrons will still flow. In simpler terms, the voltage is turned down so that the tube stops, but when the room lights are turned on, it starts again. Pretty weird.

This was called the photoelectric effect (photo meaning light, electric meaning electrons are involved, and effect meaning...well... effect).

At first no one knew what to make of this, but pretty quickly they arrived at a possible solution.

A Possible solution!

At first it seemed that the solution was simple. The electrons had ALMOST enough energy to fly across the tube, and the light provided the excess energy needed. Problem solved...but not really.

Further experimentation showed that the wavelength of light mattered. In simpler terms, the effect worked with high energy light (ultraviolet, purple, blue) but not with low energy light (orange, red, infrared). This is getting annoying. (In fact, what wavelength works depends on the metal used to make the cathode, but that is not important to understand this idea.)

Another possible solution!

UV, purple and blue light have much shorter wavelengths than orange, red and infrared light. Using the equations for the energy of light developed earlier (by Planck among others), we can calculate that UV, purple and blue light have much higher energies than orange, red and infrared do. So the explanation still works (the electrons have almost enough energy) Problem solved...except...

It turns out that red light can have a lot of energy. Specifically a VERY bright red light can have more (even much more) energy than a very dim blue light, but even a VERY bright red light will NOT make the electrons in the tube move, while a VERY dim blue light will make them move (just not as many electrons as a brighter blue light). Now what?

Now, we get to 1905 and Albert Einstein's paper.