This lesson is about the photoelectric effect. What is the photoelectric effect? Well, when photons of electromagnetic energy, visible light, X-rays, et cetera, strike a material, usually a metal surface, the electrons are released from the surface. So you have a bundle of energy here that hits a metal surface of some sort. And then because of that energy hitting, there's some type of ejected electron off of that surface.

Of course, there's also a work function, which you can see right here, that determines how much work is required to free electrons from the material. And I'll get into that in a little bit. One other thing you want to know here is that photosensitive surfaces are surfaces that are prone to have the photoelectric effect. And photoelectrons is just a fancy way of saying electrons that are emitted from a surface because of the photoelectric effect. So don't get too hung up on photoelectrons. They're just electrons.

The photoelectric effect. The work function of the material, denoted by this symbol right here, this Greek symbol, determines the minimum amount of energy needed to free electrons from the surface of the material. If the energy of the incident photon exceeds the work function, electrons are ejected. So in other words, if the energy is greater than the work function, then electrons are going to be released from the surface. And then the ejected electrons are given kinetic energy equal to the difference between the photon energy and the work function.

Here is the formula of what I just described. The kinetic energy is equal to the energy of a photon, which we learned is h times f, or Planck's constant times frequency, minus the work function. So when we have energy hit the surface, the kinetic energy that is released in the form of-- or with the electron is equal to the energy of the photon minus the work function to keep it there. So whatever is left over is really the kinetic energy that it has.