system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is Particle-Wave Duality of Light?

I have read a little bit about Quantum Mechanics and this is a bizarre phenomenon(for someone who didn't take GCSE Physics) that I've come across that isn't explained very well.. So I was wondering if someone can perhaps provide me with a more comprehensive explanation with suitable analogies so that I have a better intuitive understanding of the subject.
gpt
Waves and particles are concepts that have quite different properties.

The following are simplistic representations of some of the classical properties of waves and particles:

Waves are considered to be entities that don't have a single position, but are instead spread out over an area. In addition, two waves can interfere with each other and depending on the conditions, this interference can amplify the wave or even extinguish it. A wave can even interfere with itself, for example through it's reflection (throw a pebble in a body of water and you can see the interference of the wave with it's reflection off the edge of the water).

A particle is a discrete unit. You can count them. They have a single, well-defined position and direction. While different particles may interact, they do so in an intuitive way, according to classical mechanics. A good analogy for particle behavior and interaction is a pool table, with the balls (particles) moving around and bumping into each other and the walls.

Whether light consists of particles or waves has been a back-and-forth matter in science. In the 1800s it was discovered that light is a form of electromagnetism and through the work of Faraday, Maxwell and others, light was established as a wave phenomenon.

However, in the early 20th century quantum theory reignited the notion of light as a particle phenomenon. Work by Einstein and later Compton revealed that light had properties that could be explained through particle theory, but not with wave theory.

Eventually quantum physics evolved and the differences were reconciled with the conclusion being that light exhibits properties of both waves and particles. Depending on exactly what you're looking at, you'll find light behaving like a classical wave, like a particle or like something that's neither of those two.

For example, in the dual-slit experiment, a light source is directed at an opaque surface with 2 parallel slits in it. The light passes through the slits and is projected on a surface behind it. Doing this reveals interference patterns that are consistent with classical wave theory, but can't be explained with pure particle theory.

On the other hand, in some instances, light can be measured not as a continuous wave, but instead as discrete units of energy, as if they were separate particles. One instance of this is the photoelectric effect, a phenomenon where certain materials emit electrons when exposed to light. This phenomenon was first explained by Albert Einstein (for which he won the Nobel prize), by considering light as discrete "quanta" of energy.

So there you have it. Particle-wave duality of light means that light behaves as both particle and wave, depending on the situation. However, classical particle and wave theories are insufficient to completely describe light, which is where quantum mechanics comes in.

And as an encore: It's not just light that exhibits particle-wave duality. It's everything. The aforementioned dual-slit experiment also works with electrons, for example. In general, the heavier/bigger something is, the less pronounced its wave aspects are, which is why we don't have to consider the wave properties of macroscopic objects at all.