Light is an electromagnetic wave, and as for any wave its velocity is determined by the *medium* in which its propagating.

Sound waves can only propagate in material medium (in gases, liquids, solids, plasmas etc. but not void), and the velocity of the wave will depend on some physical properties of the medium : its density, temperature, or other more complex quantities (such as adiabatic index for gases, and Young modulus for solids). Hence, if the medium's properties are known, the speed of sound can be calculated theoreticaly. The reciprocal method has been used to determine the Earth's interior: by "listening" to sound waves (generated by nuclear explosions) from the surface of the Earth, it has been possible to calculate their velocities, and determine physical properties of the material composing Earth's interior. On a microscopical level, sound waves are small vibrations of atoms/molecules that make the medium in which they propagate, for this reason sound cannot propagate in space.

Light, or electromagnetic waves, are oscillations of the electromagnetic field that exists independently of matter. For this reason, light can travel through space (i.e. in void). In the theory of special and general relativity, the speed of light is a universal constant c=300.000 km/s (with some rounding). However, light interacts with matter, and this interaction results in the modification of the apparent speed of light on a macroscopical (human) scale. In that case, as for sound waves, the apparent speed of light will be changed depending on the medium physical properties, but in a different fashion than for sound waves. One of these properties is the relative dielectric constant of a medium (an adimensional number of value always greater than 1), and often the speed of light in the medium is simply c divided by the square root of the relative dielectric constant. So the new velocity is always smaller than c.

In the case of visible light and translucent mediums, for convenience we define the "refractive index" n of the medium as the square root of the relative dielectric constant (a value still adimensional and equal or greater than 1). In that case, the speed of light in the medium is c/n, and n is usualy measured experimentaly for various mediums. Of course, the value of n depends on the material used (and is even affected by the wavelength of the light propagating), but values of n are well tabulated. For example, n=1,33 for water, n=1,5 for glass and n=1,0003 for the air in the atmosphere (at standard pressure and temperature).

On an other planet, the speed of light would depend on the refractive index of the medium composing the planet (a gas, liquid or solid, as long as its transparent). Currently, our understanding of these phenomenons are good enough to calculate theoreticaly the speed of light on an other planet if we know the composition and properties of that planet.