1. Jul 25, 2012

### spaghetti3451

Light as a wave: Interference and diffraction of light indicates light is a wave. Other waves like sound or water waves require a medium to propagate. The speed of the wave is deﬁned relative to this medium. So what is the light’s medium? And can we measure our speed relative to the medium by measuring the speed of light?

At Einstein’s time it seemed obvious that there was such a medium—called “LuminiferousÆther ”—pervading the Universe. But its nature was very controversial. On one hand a wave is a perturbation of the æther and its frequency increases with the force which restores the equilibrium. To accommodate the very large frequencies of visible light the interaction between the medium and the light must be very strong. One the other hand the æther must be completely transparent to matter, allowing the earth to travel trough it without affecting it.

I am having trouble understanding how the frequency of a must increase with the force which restores the equilibrium and why to accommodate the very large frequencies of visible light, the interaction between the medium and the light must be very strong.

I also find it difficult to see how this contradicts the fact that the æther must be completely transparent to matter, allowing the earth to travel trough it without affecting it.

Any help would be greatly appreciated.

2. Jul 25, 2012

### ghwellsjr

Nowadays, even for those who promote an æther, the only characteristic they care about is that the propagation of light is c with regard to it. Since Special Relativity postulates that the propagation of light is c in any inertial reference frame, it kind of makes concerns over the issues you are raising a moot point.

3. Jul 25, 2012

### Bill_K

The idea behind the luminiferous ether was that light is analogous to elastic waves in a medium, and so each vibration required the ether to be pulled back and forth by the interaction. Consequently, the faster the vibration the greater the acceleration, and thus the greater the force required.

On the other hand there were not the nonlinear effects that one might expect from a strong interaction -- dispersion for example. Or scattering, or harmonic generation.

4. Jul 25, 2012

### Staff: Mentor

Consider simple harmonic motion:

$$x = A \cos (kx - \omega t)$$

1. Calculate the acceleration by taking the derivative twice. Notice how it depends on ω.

2. What does that tell you about the force which produces that acceleration?

5. Jul 26, 2012

### HallsofIvy

Staff Emeritus
Light's "medium" is the Electro-magnetic field.