# What is a medium?

I was reading this and in there it states that

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical substance [luminiferous aether(https://en.m.wikipedia.org/wiki/Luminiferous_aether) proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the Michelson–Morley experiment.

Which leads to the question the following questions:

1) "What is a medium?"

2)What is the physical notion behind it?

$$and$$

3)What are the kinds of it?( I know of one kind 'the material medium ' ,this provokes me to ask this.)

• For example, the medium for sound waves is the air, and the medium for water waves is a body of water such as a lake. I think it used to seem quite strange to think that electromagnetic waves could exist without some underlying medium (analogous to air). – littleO Oct 20 at 20:39
• Heh heh. It's bigger than a small but smaller than a big. :^) – puppetsock reinstate Monica Oct 22 at 20:07

1) "What is a medium?"

The historical way waves have entered in the world of physics is through cases of small (linear) perturbations of physical systems/ The "medium" is the physical system. One ore more physical quantities of an extended system (medium) may be described by a wave-equation. Examples may include:

• local pressure or density of a fluid;
• the local height of the liquid-air boundary in the presence of a gravitational field;
• departure from equilibrium configuration of a stretched string;

In these examples the medium is respectively: the fluid, the free surface of the liquid, the string.

2)What is the physical notion behind it?

In all those cases, the quantity exhibiting wave-like motion is a physical property of a physical system made of matter. This explains the reason nineteenth century physicists tried hard to find a material system underlying electromagnetic waves. It required some effort to get convinced that in the case of electromagnetic waves is the electromagnetic system which has to be considered the physical system and that there is no experimental basis justifying the existence of a physical system carrying a property called electromagnetic field.

3)What are the kinds of it?( I know of one' the material medium ' ,this provokes me to ask this.)

See above.

IMO this is a language question, not a physics one. The relevant senses from OED are
5.a. "An intervening substance through which a force acts on objects at a distance or through which impressions are conveyed to the senses",
5.b. "a pervading or enveloping substance", and
4.a. "an intermediate agency, instrument, or channel; esp. a means or channel of communication or expression".

The article isn't using "medium" in any special physics-specific way that isn't explainable by one of those; it's just expressing the idea that a wave needs "something to wave", some substance which carries the wave from place to place.