Question

I know that the existence of the ether has been disproved in multiple ways, and I'm pretty sure that waves need a medium to go forth. So how does light propagate when it's acting as a wave?

Answer 1

it created its own medium... its electric and magnetic fields

Answer 2

In classical physics, light is described as a type of electromagnetic wave. The classical behaviour of the electromagnetic field is described by Maxwell's equations, which predict that the speed c with which electromagnetic waves (such as light) propagate through the vacuum is related to the electric constant ε0 and the magnetic constant μ0 by the equation c = 1/√ε0μ0.[45] In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum.
Extensions of QED in which the photon has a mass have been considered. In such a theory, its speed would depend on its frequency, and the invariant speed c of special relativity would then be the upper limit of the speed of light in vacuum. No variation of the speed of light with frequency has been observed in rigorous testing, putting stringent limits on the mass of the photon. The limit obtained depends on the model used: if the massive photon is described by Proca theory, the experimental upper bound for its mass is about 10−57 grams; if photon mass is generated by a Higgs mechanism, the experimental upper limit is less sharp, m ≤ 10−14 eV/c2 (roughly 2 × 10−47 g).
Another reason for the speed of light to vary with its frequency would be the failure of special relativity to apply to arbitrarily small scales, as predicted by some proposed theories of quantum gravity. In 2009, the observation of the spectrum of gamma-ray burst GRB 090510 did not find any difference in the speeds of photons of different energies, confirming that Lorentz invariance is verified at least down to the scale of the Planck length (lP = √ħG/c3 ≈ 1.6163×10−35
m) divided by 1.2.

Answer 3

Light is a self propagating wave so it can travel through any medium, even vacuums! Since Maxwell discovered that light is an EM wave, we know it is self-propagating because of EM induction. The light source create a temporary (and CHANGING, very important) electric field. As the E field diminishes this change in flux creates a magnetic field which also diminishes. This changing M field creates the electric field further down, hence it is a self-propagating wave that needs no medium!