In 1810, François Arago realised that variations in the refractive index of a substance predicted by the corpuscular theory would provide a useful method for measuring the velocity of light. These predictions arose because the refractive index of a substance such as glass depends on the ratio of the velocities of light in air and in the glass. Arago attempted to measure the extent to which corpuscles of light would be refracted by a glass prism at the front of a telescope. He expected that there would be a range of different angles of refraction due to the variety of different velocities of the stars and the motion of the earth at different times of the day and year. Contrary to this expectation he found that there was no difference in refraction between stars, between times of day or between seasons. All Arago observed was ordinary stellar aberration.
In 1818, Augustin-Jean Fresnel examined Arago's results using a wave theory of light. He realised that even if light were transmitted as waves the refractive index of the glass-air interface should have varied as the glass moved through the aether to strike the incoming waves at different velocities when the earth rotated and the seasons changed. Fresnel proposed that the glass prism would carry some of the aether along with it so that "..the aether is in excess inside the prism". He realised that the velocity of propagation of waves depends on the density of the medium so proposed that the velocity of light in the prism would need to be adjusted by an amount of 'drag'.
According the Wikipedia article on Augustin-Jean Fresnel, Fresnel worked on his theory of light from 1816 to 1821.
The phenomenon of aberration became a driving force for many physical theories during the 200 years between its observation and the conclusive explanation by Albert Einstein.
The first classical explanation was provided in 1729, by James Bradley as described above, who attributed it to the finite speed of light and the motion of Earth in its orbit around the Sun.3 However, this explanation proved inaccurate once the wave nature of light was better understood, and correcting it became a major goal of the 19th century theories of luminiferous aether. Augustin-Jean Fresnel proposed a correction due to the motion of a medium (the aether) through which light propagated, known as "partial aether drag". He proposed that objects partially drag the aether along with them as they move, and this became the accepted explanation for aberration for some time.
My first question is as follows. Why did Fresnel only come up with partial aether drag hypothesis to explain Arago's results? His primary motivation should have been the earlier problem of aberration and his theory should have focused on it. The aberration problem was also resolved using the partial aether drag hypothesis.
My second question is as follows. Fresnel got involved in optics around 1814 but before that Young, after his experiments around 1803, has already revived an interest into wave theory of light. Had both Young and Fresnel assumed a completely stationary aether before Frsenel came up with his partial aether drag hypothesis?
Fresnel's partial aether drag hypothesis suffered from two problems.
- It was already known in the 19th century, that partial aether dragging requires the relative velocity of aether and matter to be different for light of different colours – which is evidently not the case.
- Fresnel's theory of an (almost) stationary aether predicts positive results by experiments which are sensitive enough to detect second order effects. However, experiments such as the Michelson–Morley experiment and the Trouton–Noble experiment, gave negative results within their margin of error and are therefore considered refutations of Fresnel's aether.
In my opinion, the color problem was more important because 'second order effects' problem only got tested later. Did ever anyone try to resolve this color problem with Fresnel's partial drag aether hypothesis? How could this hypothesis become so popular when it couldn't explain a rather 'simple' problem.
George Stokes came up with a competing complete aether drag hypothesis around 1845.
For George Stokes (1845) the model of an aether which is totally unaffected or only partially affected by moving matter was unnatural and unconvincing, so he assumed that the aether is completely dragged within and in the vicinity of matter, partially dragged at larger distances, and stays at rest in free space.
Was this, "unnatural and unconvincing", the only motivation for Stokes to come up with the complete aether drag hypothesis? Was the color problem one of the reasons as well?
Could you please help me with these queries? Thank you.