✔ 最佳答案
A uniform medium is one that is translational symmetric. This means the medium is the same everywhere, hence the name uniform.
Vacuum, being translation symmetric microscopically, is an uniform medium. A perfect crystal, being translation symmetric macroscopically (up to unit crystal spacing), is also an uniform medium. Pure glass is uniform, though not being a crystal, it still has a length scale for which macroscopic translation symmetry is presence.
Actually, wave speed does depend on frequency. The dependence is most significant when the translation symmetry length scale is close to the wavelength (to be precise, when the medium plasma frequency is close to the wave frequency). For example, the refractive index is given by
n = speed of light in vacuum / speed of light in medium
And we know the refractive index of glass depends on the color of light. This is why a glass prism splits white light into a spectrum.
Wave speed - frequency dependence is only insignificant when translation symmetry length scale of the medium is much longer or much shorter than the wavelength, i.e. when plasma frequency and wave frequency differs a lot.