 Photoelectric effect is a phenomenon in which light is able to eject electrons from matter. To observe the photoelectric effect you need a light source. It should emit only one color, for example only red light. Then you need a plate capacitor, which consists of two metallic plates, called electrodes. One electrode is positively charged, the other one negatively charged. For the photoelectric effect it is useful to be able to change the polarity of the plate capacitor. And finally you need a voltmeter to measure the voltage between the two capacitor plates, and an emmeter to measure the electron current between the plates. The revolutionary aspect of the photoelectric effect was that Einstein assumed that light is not wave-like, but particle-like. It consists of many light particles called photons. Each photon carries a certain energy, we call it WP, which depends only on the frequency f of the light. To determine the energy of a photon you multiply this frequency by the Planck's constant h. This is a physical constant which has the value 6.6 times 10 to the power of minus 34 joule seconds. In most cases you know the wavelength lambda of the light, instead of the light frequency. You can also express the photon energy with a wavelength h times c divided by lambda. c is the speed of light with the value 3 times 10 to the power of 8 meters per second. For example if you use the wavelength of 780 nanometers for the photoelectric effect, which corresponds to a red light, then the energy of a single photon is 2.5 times 10 to the power of minus 19 joules. Such a photon flies with the speed of light and hits the metal plate. If the energy of the photon is too small, that means you use too small light frequency, then nothing happens. But if the energy of the photon is high enough, then the photon knocks an electron out of the plate. The energy necessary to eject an electron is called the work function W. Only when the photon has an energy greater than the work function energy, then it can knock out an electron out of the plate. You can determine the work function easily because you know how to determine the photon energy. If you increase the light frequency, you will eventually arrive at a photon energy that is sufficient to eject an electron. This light frequency used, above which the electron is ejected, is called the threshold frequency F0. Sometimes it is called a cutoff frequency. If you multiply it with a Planck's constant, then you get the required energy of a photon. You get the work function W. As I said before, if you don't know the frequency, but the wavelength, then you can also calculate the work function with h times c divided by lambda zero. Here lambda zero is the threshold or cutoff wavelength. The work function depends, of course, on the material used for the electrode. For example, it is easier to knock out an electron from a natrium plate than from an aluminum plate. For a natrium plate, the photon must have at least 3.6 times 10 to the power of minus 19 joules. For an aluminum electrode, the photon must have at least 4.2 electron volts. I use different units to show that energy is often expressed in electron volts. You can easily convert joules into electron volts and vice versa. If the energy is given in joules, then divide it by the value of the elementary charge 1.6 times 10 to the power of minus 19 to get the value in electron volts. If the energy is in electron volts, then multiply it by the value of the elementary charge to convert it into joules. After a photon has overcome the work function of the plate and knocked out an electron, the electron leaves the plate with a certain velocity v. So the electron has a kinetic energy, let's call it WKIN. If we know the velocity of the electron, we can calculate it quite easily, namely with one half mv squared. The mass of the electron has the value 9.1 times 10 to the power of minus 31 kilogram. But where does this energy come from? It cannot have been created out of nothing. Let's look at the conservation of energy. We have used the energy of a photon, only this energy is available to us. A part of it was used to overcome the work function. And what if the photon had a larger energy than the work function? Where did this remaining energy go? Exactly this remaining energy is the kinetic energy of the electron. So according to the conservation of energy, we have derived a formula for the photo electric effect. Photon energy equals kinetic energy plus work function. This is the so-called Einstein formula for the photo electric effect. How do we even know that an electron moving out of the plate at some velocity? Of course, we can't see it with our naked eyes. This is where the plate capacitor comes in. We switch on the plate capacitor in such a way that the plate which is irradiated with light is positively charged, and the opposite plate is negatively charged. As a result, ejected electrons are slowed down because they are repelled by the opposite plate. We can increase this repulsion by increasing the voltage between the plates. When we set the polarity of the capacitor in such a way so that the electrons are slowed down, we call the voltage as breaking or stopping voltage. If the voltage is not large enough, then our emitter will show some non-zero value because electrons are able to reach the opposite plate. But when we set a voltage value, let's call it u0, at which the current has dropped to zero, then no electron arrives at the opposite plate. This value of the voltage at which the current has dropped to zero is very important. At this value we have managed to bring the kinetic energy of the electron to zero. When the electron traveled from one plate to the other, it converted all of its kinetic energy into potential energy at this voltage value u0. To calculate this potential energy using the stopping voltage, you just have to multiply u0 by the charge of the electron. So we can express the Einstein formula with the help of the stopping voltage like this. We can illustrate the photoelectric effect in an energy frequency graph. On the y-axis we plot the kinetic energy of the electron, and on the x-axis they use light frequency. If you start in an experiment at a small value of the light frequency f and it increases slowly, you will see that the kinetic energy increases linearly with the frequency. Draw the line a little further and you will get the y-axis intercept corresponding to the work function w. The x-axis intercept is the threshold frequency f0. Here the electron has no kinetic energy but was just ejected. A remarkable aspect of this linear relation is that the slope of this straight line corresponds to the Planck's constant h. So you can use the photoelectric effect to experimentally determine one of the most important physical constants of quantum mechanics.