I’ve seen one multiple educational videos that draw light as two orthogonal waves traversing through space. What is that trying to represent?What exactly is going up and down?
Furthermore, why do people often make the analogy that light acts like a ripple in a pond?
In: Physics
If I hold a magnet up to some conductive wire and move it around the right way, it’ll make an electric current in the wire. If I run electric current through a wire, it’ll create a magnetic field around the wire
So, magnetism can create electricity, and electricity can create magnetism (this is because they are really two sides of the same coin, but that’s a different conversation). But if that’s the case, then what if you had electricity create magnetism, which then creates electricity, which then creates magnetism, which then creates electricity, etc.? Well, that’s what light is.
Here’s a well done, more in depth explanation by a professional physicist
So specifically to the 2 orthogonal waves. That boils down to 2 if maxwell’s laws.
One says a changing magnetic field creates an electric field.
Another says a changing electric field creates a magnetic field.
Well when you go from no magnetic field to having one that’s a change so now you create an electric field and that’s a change so that creates a magnetic field which is also a change and on and on. Light is a self propagating electromagnetic wave. In the illustration one part they are showing you is electric field component and the orthogonal sine wave is the corresponding magnetic field component.
There is an electromagnetic quantum field (EMQF) permeating all of space. Electric fields and magnetic fields are classical representations of interactions involving this EMQF.
Light is a fluctuation in this EMQF, and this representation explains the photoelectric effect (which isn’t explained in classical electromagnetism).
If you think of a flash of lightning, it’s a lot like a rock tossed into a still pond. This large electric current agitates the gas atoms in the atmosphere, and when they relax they release light in all directions, like how the rock sends waves in all directions on the pond. The surface of the pond is 2D, and the EMQF is 3D, but otherwise it’s similar.
The analogy of ripples in a pond is bc thats just how waves behave. Light is an electromagnetic wave, traveling through space. The important difference is that light does not need a medium to travel. In a pond, the waves are movement of the water. Light is just a wave itself, not a wavey motion of any medium. Thats why light can travel through vacuum.
The two orthogonal waves are one electric wave and one magnetic wave. What’s going up and down is basically the electric and magnetic force. So an electric force can be positive or negative (same with magnetic). So as the wave travels through space, the electric and magnetic forces change magnitude and direction and when you plot that out you get these wave patterns.
The pond analogy is because all waves act in similar ways. If you drop a stone in a pond, you’ll see waves shoot out from the point where you dropped the stone where the water can be at the crest or the trough of the wave. If you create light and fire it out from a single point, the electric and magnetic forces would create this wave pattern in the same way.
Think of dropping a stone in a pond – you get waves of water moving outwards. These transmit energy from the source (the centre) outwards.
Light can be thought of as a very similar wave moving outwards from the light source and so transmitting energy away from the source. It is this energy we capture when that wave arrives at a solar panel.
Now water waves move up and down vertically in a gravitational field, which is why we call them gravity waves (we do, honest). Think of a gravitational field as being an area of space where objects with mass are pushed in some direction, specifically down. Light waves move up and down in an electric field, and also in a magnetic field. For slightly obscure reasons these fields exist at right angles to each other. Perhaps the best way is to think of electric fields (that push electrons around) and magnetic fields (that push magnetic materials around) are just two different ‘directions’ of a single electromagnetic field.
So water waves move through a gravitational field, and light which is an electromagnetic wave moves through an electromagnetic field.
It’s electric and magnetic fields vibrating together in unison. If you simply jerk an electron (which is a particle with a negative electric charge) back and forth periodically it will create a changing electric field which will induce a changing magnetic field which will induce a changing electric field and so on. You have an electromagnetic wave. This happens in an AC generator, for example, although the resulting wave is very low-frequency (50 or 60 waves per second usually).
A moving electric field produces a magnetic field at right angles to it. A moving magnetic field induces an electric field perpendicular to it.
Each feeds each other moving electric field —> moving magnetic field —> moving electric field.
It’s the original (and only) bootstrap process.
An electric field is a map of electric potential (volts) in space. If you have a charge (like an electron) the field is negative near the negative electron and slowly goes more positive as you move away. If you move the electron around then the field moves with the electron.
A pond surface is a map of the gravitational potential of the surface over a flat plane. If you push down with a stick, the water molecules at the stick go down and have a lower potential. If you move the stick around then the spot with low potential moves with the stick.
In both cases, if you move fast enough you notice it takes time for the potential to “catch up”. In water this creates waves. In space this creates, well, waves that seem a lot like water waves.
This electric field bobbing up and down is light. When an electric field moves you get a magnetic field whether you want it or not. And that electric/magnetic bobbing up and down is light.
I will add that there is a deep connection between electric and magnetic fields and it is more complete to call them the same thing (electromagnetic…). But you can learn that a bit later. And that rabbit hole will get you to Einstein’s relativity.
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