So I understand that light is energy and can be modelled as a wave, where different frequencies are perceived as different colours. All good. But I don’t really understand that. I can understand sound because you can view it as changes in air pressure – and that can explain how sound travels and how it is perceived. But I can’t find anything similar intuitive explanation of how light works – how it is physically manifested, how it travels, and how it can be understood in terms of a wave. Any explanation I can find assumes that you already understand the light as waves concept.
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If you have two magnets that repel each other but aren’t touching, then push one and the other one moves. How? a ‘wave’ traveled between the two.
If an electron wiggles, it can make another electron wiggle in response (with a wave carrying the energy from one electron to another). If the electron that it wiggles in response is in your retina, you’ll interpret that as light.
If you have a very sensitive pressure meter (we call that a microphone) you can measure wiggles in pressure, and we call that sound.
If you have a very sensitive electric meter (we call that a rod or cone in your eye, or a CCD in a digital camera), you can measure wiggles in the electromagnetic field, and we call that light.
You are asking questions that have troubled the greatest minds for centuries. And we do not have all the answers. We can say that light is a manifestation of electromagnetism in the form of a wave. The force it can apply to a target charged particle is electromagnetic in nature. It basically accelerates charges that are hit by the wave. If you want to go deeper than this, you could start by trying to understand how an antenna works and maybe look at how our eyes detect this kind of wave. There is a good video on yt for the latter phenomenon: https://www.youtube.com/watch?v=NjrFe7JHY1o
As far as intuitive explanations go, there is not much to go on. Maxwell’s equations surely do not qualify. But this is most of the knowledge we have.
To get a perfectly complete understanding of light as a wave, you would need to understand college-level physics pretty well. I’m going to try to cover it in a simple but accurate way.
Sound is a wave in the sense that, when air moves to the side, it pushes on the air in its way and generates pressure, which causes that air to move and generate pressure next to *it*, and so on. You get a small zone of pressure and movement which propagates itself; even though the air doesn’t move much your little pressure zone travels quite quickly.
But you knew that. I’m just recapping to compare to light.
Sound is formed by the relationship between movement and pressure in the air. Light is instead formed by the relationship between electric and magnetic fields. Take a basic radio antenna. It moves electrons up and down, creating an electric field that quickly changes directions. This forms a magnetic field alongside it, and the two fields sustain one another as they move outward just like the pressure and movement of sound do.
The answer is…light isn’t a wave. It’s just wave-like.
At least, it’s not like what you might picture when you think of a wave, like a wave on the ocean or a sound wave.
For centuries, scientists struggled with this. Experiments showed that light had frequencies that could even interfere with each other, just like sound.
But if light is a wave, it’s a wave *in what?* Ocean waves are carried by water. Sound waves are carried by air. But light happily travels through outer space, where there’s nothing. So what kind of wave is carried by nothing?
For a while they thought there must be something called the “luminiferous aether” (light-bearing heavenly air) that pervaded space. It was supposed to have contradictory properties – it was extremely rigid, to transmit light waves so fast, and yet completely intangible, because physical objects aren’t affected.
They tried a lot experiments to find the aether and nothing worked (until Thor 2).
So scientists just gave up.
Nowadays, we think of light as an electromagnetic wave in “space”, but this isn’t very helpful to visualize what’s happening.
Especially because, half the time, light acts like a particle! It comes in little units (photons) and with a solar panel you can turn it into little units of electricity (electrons)? Now what? Wave AND particle?
It’s better to think of waves as a *metaphor* for how light works. Light works the way light works. It doesn’t have to behave like anything we are familiar with in the everyday world. Some of the math looks like the same math we use for sound waves. Some of the behavior looks like waves to our intuition.
But light just acts like itself.
Light is modeled as both a particle and a wave depending on the circumstances. Certain experiments such as the double slit interference experiment show that light behaves like a wave. Others will show that light behaves as particles, such as the photoelectric effect and black body radiation. We don’t have a single unified theory that is able to explain both the wave and particle situations. This wave-particle duality is eventually what lead to quantum mechanics, which introduces statistics theory into the mix.
Anyways, light can be thought of as a beam of particles for the most part. Our current model of light as a wave shows that light isn’t a mechanical way, but rather is an electromagnetic wave with oscillating magnetic and electric fields that are perpendicular to each other.
It sounds like you’re asking what exactly the electromagnetic field is. We describe it using vector fields.
Imagine a fluid, like water or air – except instead of being made of atoms, it’s our old conception of fluids, a true continuum with no smallest parts. At every infinitely small point, the fluid will be flowing in some direction with some velocity. You can represent this with an arrow, except when you try to draw it you can only put down finitely many arrows. However, the fluid flow doesn’t actually have gaps. There are infinitely many points infinitely close together, each with its own direction and velocity, and all together they describe the flow of fluid. Mathematically, we describe this as a function – if you give the function some point in 3D space, it will give you back an arrow describing how fluid is flowing at that point.
This is a vector field. The electric and magnetic fields are also vector fields, but it’s not a vector field *of* anything. Instead, you can think of it as a property of space. Space itself, with no matter in it, ‘knows’ about these two vectors fields – the magnetic and electric fields. Every point in space is pointing in two directions, with their two corresponding magnitudes. The electromagnetic field is the combination of these two vector fields, since it turns out they are deeply related, and it makes more sense to think of them as a single thing instead of two things that interact.
It may seem weird to say that an abstract mathematical function is a thing that exists, but that’s the state of physics these days, and it has been ever since Newton. Instead of nice contact forces between objects, we have to use abstract mathematical entities to create physical models, and they create a rich framework that we can use to explain why human-understandable objects behave the way they do.
An electromagnetic wave is an oscillation in the electromagnetic field. The loose description is that, when the electric field changes, it creates a perpendicular magnetic field, and when the magnetic field changes, it creates a perpendicular electric field. So if you create a change in either one, they will start oscillating back and forth, and this oscillation travels in a direction perpendicular to both of the fields (e.g. electric field is oscillating left/right, magnetic is oscillating up/down, and the direction of travel will be toward you or away from you).
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