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[V][/B] = c/n + αv,

[V][/A] = c/n - αv,

where n is refraction index of the water, and α is the Fresnel's drag coefficient, equals to 1 - 1/n², v is the velocity of the water, and c is the velocity of light in vacuum.

Nowadays we can reinterpretate this experimental result with Einstein's transformation of velocities.

Let's consider two reference frames: the laboratory frame, and the water frame, moving in respect to the first. In the water frame the velocity of light is c/n. In the lab frame the velocity of light must be:

[c][/lab] = (c/n + v)/(1 - v/nc)

If we expand this expression and neglect terms of the order of (v/c)2 and higher, we obtain exactly the same results as predicted by Fresnel's theory.

Ok, so far so good.

But one may ask: "The principle of relativity teaches us that light moves with the same speed, no matter the frame of reference (lab frame, water frame, whatever). So, how can we explain the Fizeau's experiment?"