Now Einstein theorized that if a photon met an energy requirement and struck a metal surface, then electrons could be ejected. This is known as the photoelectric effect. So here if we look at it, we have a metal surface here, and on this metal surface we have free flowing electrons in green.
Now we're going to say that to this metal surface I'm going to strike it with some form of energy, here represented by energy of a photon. If it hits the surface with enough energy and enough force can dislodge one of these electrons, that electron can then convert that energy X excess energy into kinetic energy and propel itself further away from the metal surface. So this is what we call our kinetic energy.
Now for this to happen, we have to realize that when the energy of the photon or light source is greater than the energy of the surface which we're going to abbreviate as BE, then we can eject an electron. But if the energy the light source is not greater than the energy of the surface of the metal, then electron will not be ejected from that metal surface and it'll stay there.
So this is basically the idea when it comes to the photoelectric effect. Now let's click on to the next video and go a little bit more in detail about what exactly is energy KE versus energy BE.