As fascinating as I find learning about, well, just about anything, I know that most people don’t necessarily share my fascination with, for instance, how microwave ovens work, the inner workings of ballpoint pens, the physics underpinning cellular communications, or why there are at least three different definitions of north. Perhaps my favorite part of the ease with which the internet enables us to access such a vast quantity of information is how quickly and readily I can find information on all of the different topics that come up in a given day that I want to learn about in far more detail than anyone has the patience to offer me. The truth is that our modern systems are astonishingly complicated, and coming to a detailed understanding of all of them is probably beyond the capacity of any one individual in a single lifetime.
There are certain principles, however, that I have found underpin an astonishing number of our modern systems, and gaining a thorough understanding of a principle like that can enable you to understand or surmise how so many different things work. One of those, which is what we will be discussing today, is the photoelectric effect. It seems like at least once a week I come across some new piece of technology that leverages the photoelectric effect in a completely new or different way, and increasingly I marvel at how such a relatively simple principle underpins so much of our modern world. So let’s talk about the photoelectric effect.
Although Einstein is today mostly remembered for his theories of general and special relativity, which describe the grand motions of the cosmos and the behavior of spacetime itself, he actually won his Nobel prize for work in particle physics – specifically, for the formulation of the photoelectric effect. At its simplest, the photoelectric effect describes a relationship between electrons and photons: that photons can impart their energy to electrons, and that electrons can emit energy in the form of photons. In other words, when a particle of light encounters a “particle of electricity” (this is not really true, but will work for this very high level description), it is absorbed, and increases the “electric particle’s” energy. When an “electric particle” loses energy, it emits that energy in the form of a light particle – that is, a photon.
To gain a deeper understanding of what’s really going on in the photoelectric effect, and to start to appreciate the mind-boggling number of ways in which it is currently being leveraged, we need to establish some principles. First, let’s talk about electrons. Electrons, which I previously and very inaccurately referred to as “electricity particles,” are fundamental pieces of matter (they cannot be split into constituent particles, at least as far as we know today) possessing a negative electric charge (no need to go into what a negative charge really is right now – that is far beyond the scope of this discussion). In an atom, some number of electrons will typically “orbit” about a nucleus composed of an equal number of positively charged matter particles called protons, and sometimes neutrally charged matter particles called neutrons. Both protons and neutrons can be further divided into smaller particles, but that is again beyond the scope of the present discussion.
The most accurate model to date of this atomic interaction is the electron cloud model, which states that electrons have no defined position or velocity until measured, but instead exist as probability waves, and the regions in which a given electron, based on the electron’s native energy (think of it as how fast the electron is vibrating for now, although this isn’t really what’s happening), is most probable to exist are known as shells. Confused? You and everyone else: quantum physics is terribly confusing. Fortunately, we don’t need to use the electron cloud model to understand the photoelectric effect; we can instead use the Bohr model, which is much simpler and more intuitive – it’s probably the atomic model that you imagine when you think about atoms. I only brought up the electron cloud model so that you would be aware that the following discussion should be considered more like an analogy than a description of reality.
In the Bohr model, the positively charged atomic nucleus is orbited by electrons that exist along discrete paths, or energy levels, represented by differing distances from the nucleus. Nominally, there might be, say, two electrons at energy level n=1, and another at energy level n=2. The discrete nature of those energy levels is important, as we will see shortly; although electrons can exist at any energy level, they cannot exist between energy levels – instead, they jump instantaneously from one energy level to another through a phenomenon called quantum tunneling that is also far beyond the scope of this discussion. Think of it like this: the electron can be on the first floor of a building, or the second floor, or any other floor, but you will never find it on the stairs in between floors.
Now that we understand enough about electrons, we can move to photons, which we will talk about much more briefly. Photons are packetized electromagnetic energy – that is, they are particles of light (hence the old joke: a photon walks into a hotel, and the receptionist asks if it needs help with its luggage. It replies “no, I’m travelling light”). They have no mass, and travel at, logically enough, lightspeed. The amount of energy contained in a photon is represented by several factors, such as wavelength and amplitude. While fully explaining why this is would require a crash course on wave-particle duality, we can adequately say that each photon has some representative wavelength that corresponds to its energy.
Imagine that a photon of a certain wavelength is travelling along, and collides with an electron. What the photoelectric effect suggests, and what has since been proven many, many times, is that when a photon collides with an electron, it imparts its energy to the electron. That causes the electron to “absorb” the photon, and thence jump to a higher energy levels. In simplest terms, if a photon has some energy E=3 equivalent to the different between energy levels n=1 and n=4, then when that photon collides with an electron at n=1, the photon will be absorbed by the electron, and the electron will be boosted to energy level n=4. Shorter wavelengths mean more photonic energy, which will excite the electron further (send it to higher energy levels) up until the point that the electron becomes so energetic that it escapes the electrostatic influence of the nucleus and becomes a free electron.
However, the universe generally doesn’t like high energy states. The Laws of Thermodynamics, especially entropy, tell us that something in a high energy state will tend to decay to a lower energy state, and the same is true in the case of these excited electrons. Thus, we find that the inverse of the above photon absorption also occurs. Excited electrons can emit photons of wavelength proportional to the number of energy levels they fall. So our electron that was excited to energy level n=4 will naturally tend to fall to lower energy levels, and in the process might drop to n=2, emitting a photon of E=2, and then again to n=1, emitting a photon of E=1. Those photons will have different wavelengths, perhaps E=2 is green, and E=1 is red.
Before we continue, I want you to take a moment, and ponder the possible applications and implications of this effect. If it helps, you can stare at this four year propagation of the TESS P/2 lunar resonance orbit I created with STK’s Astrogator tool. You know, maybe I should do a post on that kind of orbit, because it’s really quite fascinating.
Okay, welcome back. What did you come up with? If you’re like me, the first thing you might associate the photoelectric effect with is solar panels (and by solar panels, I mean photovoltaic cells). In photovoltaic cells, photons incidence upon a surface with enough energy to entirely free electrons from their nuclei, creating an accumulation of negative charge compared to the neutral substrate. This represents a potential difference, which is a voltage. Since the potential voltage will try to discharge to neutralize, that energy can be harnessed, and thus electricity is generated from light. This is a use of the photoelectric effect.
Spectroscopy, where we study the composition of materials based on their distinct element color signatures that are a result of the photoelectric effect, is useful for everything from astrophysics to forensics. Since every element has a unique structure of nucleus and electrons, it turns out that each element also has an associated, unique, spectroscopic signature based on the most likely wavelengths of photons released when the excited electrons fall back to their ground states. It turns out that we can then derive the unique signatures of larger compositions, as well. We can use this same concept to, for instance, tell if the bits of rug fiber found on someone’s clothing are the same as those found on the murder victim.
Cameras use the photoelectric effect. The sensor collects photons, which all have some wavelength, and those photons interact with electrons in the sensor, causing them to jump to different energy levels. Depending on the energy levels, different voltages can be registered, and thus a recording of the original photons (and thus the colors) can be made.
So-called neon signs (which are rarely neon, but are instead some other noble gas, based on the color desired) are a direct result of the photoelectric effect. A current run through the gas excites the electrons in the atoms, which then fall back to ground state and emit photons of a specific wavelength. Fluorescent lights do the same thing, just with many different wavelengths so that they appear to generate white light.
Speaking of microwaves, as we mentioned in the beginning of this post, do you know what microwaves are? They’re just a specific wavelength of photon. So to generate them, we use the photoelectric effect, applying a current to a substance to excite electrons that will then fall and release a photon of the energy associated with microwaves. To make this perpetuate, we use something called a magnetron, but that’s a different discussion.
I could go on listing things that use the photoelectric effect, but I will spare you, and instead encourage you to look around and see if you can think of more things, either commonplace or not, that leverage the photoelectric effect. Post what you come up with in the comments below. Still have questions about the photoelectric effect? Feel free to post those below as well, and I will do my best to answer them, or direct you towards addition resources.