This question is triggered by information in the following article: Shahen Hacyan Refraction, the speed of light and minimal action: from Descartes to Maupertuis through many more

First some general background:

As we know, Fermat noticed that Snell's law can be accounted for if it is assumed that propagation of light in glass is slower than in air, in a specific ratio.

Newton, as part of a corpuscular theory of light, offered the hypothesis that the propagation of light is faster in glass, in the same ratio as featured in Fermat's principle, but inverted.

So we have that while Snell's law narrows things down to a particular ratio, Snell's law does not provide the means to establish whether in a denser medium propagation of light is slower, or faster.

We have: in terms of Huygens' principle: in a denser medium propagation of light must be slower.

My expectation is: over time the evidence for the wave nature of light accumulated, and presumably the physics community settled on an expectation that in denser medium propagation of light is slower.

Returning to the article by Shahen Hacyan:

According to Hacyan Maupertuis had the expectation that in denser medium propagation of light is faster.

According to Hacyan: Maupertuis felt he had dicovered an an opportunity to unify the descriptions of propagation of light and mechanics.

Maupertuis' action in modern notation:

$$ \int m \ v \ ds $$

The idea was to account for refraction of light in terms of that action.

Shahen Hacyan states that the interferometric experiment by Fizeau to probe the speed of light in water was the first time that experimental evidence was obtained showing that in water propagation of light is slower.

(To my knowledge: Fizeau's motivation to set up that experiment was to probe whether Fresnel's theory of propagation of light was correct.)

My specific question is:
Prior to Fizeau's experient, had the physics community already settled on an expectation that in denser medium light propagates slower, and if so, when?

  • 1
    $\begingroup$ They settled on it when wave optics won over Newton's corpuscular optics. As described in a recent answer, the turning point was Fresnel's and Arago's work on diffraction around 1819, and by 1823 even staunch corpuscularists like Laplace conceded. $\endgroup$
    – Conifold
    Apr 2 at 23:55
  • $\begingroup$ @Conifold The problem of light was a most confounding one. Quite likely I will be posting follow-up questions, such as whether all corpuscular theories had light propagating faster in denser medium. Anyway: it's really rather strange that history of Maupertuis' work always mention his action concept. Historically Maupertuis' action never gained traction. It got pulled into attention by the introduction of Hamilton's stationary action. $\endgroup$
    – Cleonis
    Apr 3 at 21:30
  • $\begingroup$ Well, least action also got Euler's attention prior to Hamilton, see his Methodus inveniendi. $\endgroup$
    – Conifold
    Apr 3 at 22:58
  • $\begingroup$ @Conifold Yeah, I came across that wikisource translation of the Additamentum II some years ago. That translation is particularly interesting for the following reason: The translator has made a conscious decision to allow anachronisms, such as the concept of kinetic energy, which at the time did not exist in the form $\tfrac{1}{2}mv^2$. As we know: at the time the concept of Vis Viva was in use. It's a transciption for the purpose of making Euler's intentions accessible. And indeed it has allowed me to understand what Euler's action concept is about. $\endgroup$
    – Cleonis
    Apr 4 at 18:55


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