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• Noa Feldman

4 - Who Could Ever come Up With This? or: The Photoelectric Effect

*Is it your first visit here? Welcome! My name is Noa and I am a Physics Ph.D. student at Tel Aviv University. I write here about quantum mechanics for non-physicists. No background in mathematics or physics is required to read the blog, but I highly recommend reading the first three posts before reading this one, starting here.*

At the end of the 19th century, the physics world faced a few significant and hard problems. Two of the most important problems were two phenomena, observed in experiments but cannot be explained by classical physics (which back then, was just 'physics'): first is black-body radiation, which we do not elaborate on here, and the second one was the photoelectric effect. Scientists consider these two experiments to be the thread that led to the discovery of quantum mechanics.

In the experiment demonstrating this effect, a piece of metal was radiated with light and connected to an electric circuit without a battery or any other power source. Since light is a form of electromagnetic wave, its energy makes the metal emit electrons and creates an electrical current in the circuit. We expect that the more energy this light has, the more charges can be released from the metal and more current is observed in the circuit.

What does it mean that light has more energy? The light's energy depends on two things: the intensity of light and its frequency. The intensity of light just means how strong the light is. The frequency is, for seeable light, its color, and in general, the number of full periods the light completes over the course of a meter (or any other space unit of choice).

A good analogy here is to think of coffee. The stronger the coffee is, the better job it does in waking us up. The strength of the coffee is equivalent to the frequency of light. But if we drink a lot of weak coffee, we will still become alert. The amount of coffee is equivalent here to the intensity of light. In our analogy, we are trying to wake the electrons up and make them run in the circuit. For that, we need a little bit of strong coffee or a lot of weak coffee. Either of the two will be enough, as long as the electrons get enough "caffeine" and are emitted from the metal. The more "caffeine" we give the metal, the more charges are emitted from it.

So we expect that if we keep the intensity of light (amount of coffee) constant, but increase the frequency of light (strength of coffee), the electrical current will become higher (pink and blue lines). If we keep the frequency constant and increase the intensity of light, the current will also become higher (green and orange lines).

The experiments' results were confusing: the expectations above were met, but not always. In some frequencies, the current indeed became higher as the intensity became higher. But in other, lower frequencies, the current was zero - no matter how big the intensity of light was. Even when the light's energy was huge due to the high intensity, no charges were emitted at all

Going back to our coffee analogy - the experiments' results showed that when the coffee was weak, even barrels of coffee were not enough to wake the electrons up. On the other hand, even a small glass of strong coffee was enough to release some electrons from the metal. So we get something like this:

This made no sense. Even if the coffee is weak, we can still pump ourselves with caffeine if we drink enough cups, so why is light behaving differently? Why can't the electrons use the energy given to them by low frequencies?

Einstein's solution

This peculiar phenomenon, called 'The Photoelectric Effect', was one of the biggest questions occupying physicists at the end of the 19th century. Until 1905, when Albert Einstein, back then only a clerk in a patent office in Switzerland, provided an explanation: The energy of light does not come in a continuous form. Instead, the light comes in 'packets' of energy, and the amount of energy of each packet depends on the frequency. The electrical charges in the metal interact with each of these packets separately. Therefore, if a single packet has enough energy, it will be able to wake a single electron up. Otherwise, no matter how many packets we throw on the metal - they will not be able to release the electrons from it.

These packets behave like something else we know in physics: particles. So in fact, Einstein's suggestion was that light is composed of particles, or at least of something similar to particles. Particles of light are called photons today. But back then, light was considered to be a wave, and not a particle, and Einstein's explanation of the photoelectric effect implied that this understanding of light was not accurate.

This understanding gave quantum mechanics its name - these packets are quanta of energy, The understanding that light is not a continuous thing like waves, but divided into discrete packets like particles, was the first important insight that led to the development of the entire theory of quantum mechanics.

This insight led the French physicist Louis de Broglie to think of another interesting idea. de Broglie asked: "If the light is also a particle, maybe the electron is also a wave?". Based on this assumption, he calculated what the wave-like properties of the electron are supposed to behave like, based on what was already known about light waves. This was the source of understanding that particles like electrons also behave like waves. This idea, which is considered the jump-start of quantum mechanics, is called particle-wave duality. Both Einstein and de Broglie won the Nobel Prize in Physics for these works, in 1921 and 1929, respectively.

The story above teaches us two important lessons: First, the experiments we conduct should be as simple as possible, so we can isolate our observations and pinpoint the cases where our expectations are not met. This is an important principle of science in general.

Second, note that an unsuccessful experiment, that is, an experiment whose results did not match the theoretical expectations, opened the door to a whole new field in physics, without which we would not be able to understand how crystals behave, radiation, electromagnetism, what the universe is made of, and how we can use these properties to develop new technologies. So the lesson here is that sometimes failed experiments are more exciting than successful ones. The failure to predict their results can imply that there is something new to learn from them.

(The credit for the last sentence belongs to Prof. Ron Lifshitz from Tel Aviv University, who taught me a lot of physics and also a lot on science communication).