What really is light?
Young’s double slit experiment shows us that light is a Wave, and there’s no buts about it.
Einstein: “But, but, but.. PHOTONS!”
No, and yes, there’s that whole Wave-Particle Duality dealio, but for now we can lay that aside and peer into the minds of EVERY GD PHYSICIST (in at least the 19th century) thought about light.
The experiment is pretty much as you see it shown here apart from a few points (replace “electron gun” with “candle” and add another single vertical slit screen in between, and you’re golden).
The light is emitted from the source, and if it ain’t already well-behaved, you kind of need to “smooth it out” by using the single slit screen I mentioned before. If you make that slit small (thin?) enough, it acts pretty much like a single point emitting light that is in-phase. Now, shine that light uniformly onto a double-slit screen, and watch what happens.
The light goes through the slits, and it interferes with itself, and produces what’s called an interference pattern on the detection screen. That’s exactly what we’d expect to see if light was a wave, and it is in fact a wave, as Maxwell showed with his famous Maxwell’s Equations, which describe how an oscillation in the Electric Field produces a phase-shifted oscillation in the Magnetic Field. Long story short, he found that it would oscillate at a speed: where the two constants in there are the permeability and permittivity of free space respectively. Compute that, and you get something close to , the speed of… light!
So this was the final nail in the coffin on whether light was a wave or a particle (til we decided to exhume the debate). But something was still funny about that expression. The speed of light was a constant? Maxwell’s equations were pretty well laid out, so it was pretty fucky, but Einstein played around with the idea that the speed of light was always the same, no matter how fast you are moving, and out popped the Special Theory of Relativity.