The experiment of Fizeau
Fresnel introduced the drag factor to explain
or rather to compensate for the fact, that the earth seemed to be at rest in
the ether. Obviously they were looking for proof of the absolute ether. If
there was no ether it would be inexplicable how light, fields and forces like
gravity, electrostatic and magnetic fields could be propagated through space.
The urge to prove the existence of the absolute ether was imminent. Without the
ether, physical characteristics like the transportation of light in vacuum,
became inconceivable and mysterious. Proving the existence of Fresnel’s drag
factor was necessary. Demonstrating the drag factor was not easy because of the
extreme velocity of the speed of light with approx. 300,000 km/s.
In 1851 Fizeau devised an experiment with
unknown accuracy that was able to measure Fresnel’s assumed drag factor.
Figure 6.
The experiment of Fizeau.
The light of
source is divided up into two beams. One beam goes with the flow of water and
the other beam against the flow. When the light in the water is dragged the
light traveling against the water flow will need more time to travel through
the tubes than if the light goes with the flow. This is comparable with the
time difference a swimmer needs to swim with or against the current. The
difficulty here is that the speed of the “swimmer” is so high that it takes him
only 1/100,000,000 of a second to finish. The difference between with or
against the flow is even much smaller.
Light is
vibration with a very short wavelength. Vibrations are able to fortify or tone
down each other when they meet. The difference between extinguishing and
fortifying, a bright and a dark line in the interference pattern, is about 1/40
of a millionth of a meter. The interference of light is characterized by bright
and dark stripes: the interference lines. The whole picture of dark and light
stripes is called the interference pattern.
The experiment of Fizeau is called an optical
interferometer and was devised to measure very small differences in time or
distance. The drag coefficient of Fresnel, 
, implies in the experiment of Fizeau a drag of 
interference
lines.
(l the wavelength of the light, v the flow speed of the water and L
the length). Fizeau registered during his experiment a shift of d=0.23 interference lines; which implicates a drag factor f=0.48;
the empirical value. The theoretical value of the drag factor is calculated
from 
and is f=0.43. So
within an error of approx. 10%, the experiment of Fizeau confirmed Fresnel’s
drag factor.