# 物理學 關於均勻介質(uniform medium)

Rating

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.

• A "uniform medium" means there is no spatial variation of the density of the medium. That is, the density is constant throughout the whole medium.

Velocity of a wave passing through a medium depends on the medium density. For example, the speed of sound in air is inversely proportional to the square root of the density of air.

In a uniform medium, the density remains constant, the speed of wave will be kept constant. The frequency and wavelength thus are unchanged.