eli5:What is a wave in physics?



I know this seems like something that is easily googlable and that this is a dumb question but I couldn’t find anything that explained it well enough for me to understand. I saw that photons are just comprised of waves but what are these waves made of? Are they physical? How can we see waves in the form of light if they are very small?

In: Physics

Waves are just mathematical constructs that help us “visualize” or “interpret” natural phenomena.

Take sound for instance. When it moves through the air, it doesn’t literally look like a wave that we graph with sines and cosines. What happens instead is a column of air molecules that is disturbed and starts moving sideways, collides with another column of air and pushes it forward due to built up pressure. But mathematically, we can represent this propagation with waves, because there is a frequency and amplitude.

Same goes with light. But what’s interest with light is that we can model it as a wave AND particles. Both of these models serve their purpose and help us understand how light behaves. They are just mathematical models.

In a physical medium (like air or water), a wave is a certain “energy pattern” that is expressed in a physical way, like the waves on the ocean’s surface or the back and forth movement of air molecules (sound waves). You can see what these look like – a rhythmic up and down or side to side or back and forth movement of something physical. With light/electromagnetic energy (irreducible packets of which are called photons), wave-like behavior is experienced but there’s nothing the waves of electromagnetic energy are moving in – they are just waves (or rather, they act like waves). We can’t see the waves, we can only see how light and other forms of electromagnetism behave, and that behavior can only be explained by waves. The really odd part is that light seems to behave as BOTH a wave and as a particle, depending on what you’re measuring. And it’s not that a large collection of light particles move in a wave-like way, it’s that each irreducible unit of light (a photon) is both a wave AND a particle. There’s no way to picture this or understand it intuitively, but theory and experiment after experiment establish that it must be so.

Disclaimer: I’m going to try to explain it, but getting deep into this gets surprisingly hard.

>photons are just comprised of waves

Not exactly. Light is both particles (photons) and waves, meaning that some physical phenomena involving light are due to the light as particle aspect, others are due to the light as wave aspect. That doesn’t mean photons are made of waves.

Photons are definitely physical. They are particles, they have a physical size and speed and stuff, but they are very special since, for example, they have no mass. However, yes, they are definitely physical.

Waves can be considered a way to transport energy without transporting matter.

Mechanical waves need a physical medium to transport energy from A to B. If you think any physical object as made of molecules and atoms, a mechanical wave goes from A to B by transferring energy to the first molecule it meets, which then starts to vibrate and transfer some of that energy to the second molecule and so on. You’re not transporting matter from A to B, you’re not shooting particles to produce a wave (much like a wave in a lake doesn’t actually carry water molecules from one place to another). But you’re clearly transporting energy. Sound is a mechanical wave.

Light is an example of electromagnetic wave, like infrared, ultraviolets, microwave, X-ray. Electromagnetic waves can transport energy without transporting matter (they’re waves) AND even without a physical medium in between. Sound doesn’t propagate in vacuum (there’s no molecule to vibrate), but light does (in fact, space is pretty close to vacuum).

We can see light because our eyes are basically very sensitive receivers “tuned” to the visible part of the electromagnetic spectrum (light). Much like your phone or radio can receive radio signals from far way, we can see because our eyes can receive light from far away, and we see objects scattering (reflecting) some “colors” of light while absorbing others.

* Waves are like lining up a bunch of your cousins shoulder to should and shoving the one on the end.
* That shove will cause the first cousin to push into the second cousin.
* This continues all the way down the line.
* The energy moving from one cousin to the next is a wave.
* Now say you have a neighbor friend who is a bit creepy and has a thing for that first cousin of yours in line.
* Every 100th of a second she writes down how far to the side that cousin was pushed.
* If you took all the measurements she wrote down and plotted them on a graph you would see that typical curving line that goes back and forth.
* This is a wave graph but we often times call that graph a wave as well as a shorthand.
* It turns out that a lot of things in our world behave this way.
* If you disturb them from their normal place, they not only have a tendency to disturb their neighbors, but they also tend to swing back the other way and eventually settle down back to their original position.