Video Transcript
A tunable laser is used to illuminate the surface of a metal with different frequencies of light. Above a certain frequency of light, electrons are emitted from the surface of the metal. The graph shows the maximum kinetic energy of the electrons emitted against the energy of the photons. What is the work function of the metal?
The idea here is that we have some metal surface, and photons, little packets of light energy, are incident on that metal. The horizontal axis of our graph shows us the energy of incident photons in units of electron volts. The vertical axis of our graph shows us the maximum kinetic energy of electrons that have been ejected from the metal surface. This ejection happens through what is called the photoelectric effect. When the packets of light, the photons, incident on our metal have enough energy, they’re capable of causing an electron to be emitted when the photon is absorbed by the metal.
Looking at our graph, one interesting feature is that there is a range of incident photon energies for which the kinetic energy of emitted electrons has no value. That tells us that photons with this energy range are not energetic enough to strip an electron off of the metal. We can think of this amount of energy here as the minimum photon energy required to break any bonds an electron might form with the metal. It’s only when at least this much energy is transferred to an electron that it can escape from that surface. The amount of energy that must be transferred to an electron in a metal so that it escapes that metal with a kinetic energy of zero is called the work function of the metal.
We want to use our graph to figure out the work function of our particular metal. If we look closely at our horizontal axis, we see that between two and three electron volts, there are one, two, three, four evenly spaced, smaller tick marks. Each one of these smaller marks then indicates a difference of 0.2 electron volts. So if we start right here, we have 2.0 electron volts then 2.2 and 2.4 and then 2.6 electron volts. Our graph shows us that this is the lowest amount of energy in an incident photon that can stimulate the emission of an electron. This is the metal’s work function, which we found to be 2.6 electron volts.