Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Bookmarkable meta post with links to all previous topics

Today's topic

Today's topic is the dual nature of light and matter, the wave-particle duality. It is a central concept in quantum mechanics that - as is tradition - violates common sense. I will first discuss the duality for light and then, in the next post, for matter.

The dual nature of light

In what terms can we think of light so that its behaviour becomes understandable to us? As waves? Or as particles? There are arguments to be made for both. Let's look at what phenomena we can explain if we treat light as a wave.

The wave nature of light

Let's start with an analogy. Drop two stones in a pond, imagine what happens to the ripples in the pond when they meet each other. They will interact, when two troughs meet they amplify each other, forming a deeper trough. When two crests meet they do the same. When a crest and a trough meet they cancel out.

Now if we shine light through two small openings and observe the resulting pattern, we see it's just like ripples in a pond, forming an interference pattern. When looking at the pattern formed on a screen placed at some distance from the openings, we see a striped pattern Light can be described as an electromagnetic wave, with crests and troughs. It sure seems like light is wavey! The wave nature of light allows us to describe phenomena like refraction and diffraction.

The particle nature of light

When we shine light on some metals, they will start tossing out electrons. This is called the photoelectric effect. How can we understand this process? Well we know light is a wave, so we imagine that the wave crashes into the electron that is chilling out near the surface of the metal. Once the electron has absorbed enough of the light's energy it will be able to overcome the attractive forces between itself and the positively charged atom core (remember, an electron has negative charge and so is attracted to the atom cores). So a higher intensity of light should make the electron absorb the required amount of energy more quickly. Easy, done!

However, there's something very peculiar going on with the photoelectric effect. If we shine low frequency light on said metal, no matter how intense the light, not a single electron will emerge. Meanwhile if we shine very little high frequency light on the metal, no matter how low the intensity, the electron will emerge. But how can this be? A higher intensity of light should mean the electron is receiving more energy. Why does frequency enter into this?

It seems that the electron needs a single solid punch in order to escape the metal. In other words, it seems it needs to be hit by something like a microscopic billiard ball that will punch it out of the metal in one go. The way physicists understand this is by saying light is made up out of particles called photons, and that the energy a photon carries is linked to its frequency. So, now we can understand the photoelectric effect! When the frequency is high enough, the photons in the light beam all individually carry enough energy to convince an electron to leave the metal. When the frequency is too low, none of the photons individually can knock an electron out of the metal. So even if we fire a single photon, with high enough frequency, at the metal we will see one electron emerging. If we shine low frequency light with a super high intensity at the metal, not a single photon will emerge.

So there you have it! Light is made out of particles. Wait, what? You just told us it's made out of electromagnetic waves!

The wave-particle duality of light

So, maybe light is just particles and the wave are some sort of emerging behaviour? This was a popular idea, one that Einstein held for some time. Remember the experiment where we shone light through two small openings and saw interference (commonly known as the double slit experiment)? Let's just take a single photon and shoot it at the openings! Because light is particles we'll see the photon just goes through either opening - like a particle would. Then all the non-believers will have to admit light is made out of particles! However, when we do the experiment we see the photon interfere with itself, like it was a wave. Remember this picture which we said was due to wave interference of light? When a single photon goes through the openings, it will land somewhere on the screen, but it can only ever land in an area where the light waves wouldn't cancel out. If we shoot a bunch of photons through the openings one at a time, we will see that the photons create the same pattern as the one we said is due to wave interference!

Implications

So it would seem light acts like a particle in some cases, but it acts like a wave in some others. Let's take a step back and question these results. Why are we trying to fit light into either description? Just because it's convenient for us to think about things like waves and particles - we understand them intuitively. But really, there is no reason nature needs to behave in ways we find easy to understand. Why can't a photon be a bit wavey and a bit particley at the same time? Is it really that weird, or is it just our intuition being confused by this world we have no intuitive experience with? I would love to hear your opinions in the comments!

Observing photons

To add one final helping of crazy to this story; if we measure the photon's location right after it emerges from the slit we find that it doesn't interfere with itself and that it just went through a single slit. This links back to my previous post where I described superpositions in quantum mechanics. By observing the photon at the slits, we collapsed its superposition and it will behave as if it's really located at one spot, instead of being somehow spread out like a wave and interacting with itself. The self interaction is a result of its wavefunction interacting with itself, a concept that I will explain in the next post.

Conclusion

We learned that light cannot be described fully by treating it simply as a wave or simply as a bunch of particles. It seems to be a bit of both - but neither - at the same time. This forces us to abandon our intuition and accept that the quantum world is just fundamentally different from our every day life.

Next time

Next time we will talk about the dual nature of matter and try to unify the wave and particle descriptions through a concept known as the wavefunction.

Feedback

As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

Addendum

The photoelectric effect is actually what gave Einstein his Nobel prize! Although he is famous for his work on relativity theory he was very influential in the development of quantum mechanics too.