'Treatise on Light ' Summary

Treatise on Light: In Which Are Explained The Causes of That Which Occurs in Reflection & Refraction  is a book written by Dutch polymath Christiaan Huygens that was published in French in 1690. The book describes Huygens' conception of the nature of light which makes it possible to explain the laws of geometrical optics as shown in Descartes' Dioptrique, which Huygens aimed to replace.

Unlike Newton's rival corpuscular theory, which was presented in the Opticks, Huygens conceived of light as an irregular series of shock waves which proceeds with very great, but finite, velocity through the ether, similar to sound waves. Moreover, he proposed that each point of a wave front is itself the origin of a secondary spherical wave, a principle known today as the Huygens-Fresnel principle. The book is considered a pioneering work of theoretical and mathematical physics and the first mechanistic account of an unobservable physical phenomenon.

Huygens' ideas on light originated in his unpublished work on the properties of lenses and their configurations entitled Dioptrica, which began in 1652. In 1672, the problem of the strange refraction of the Iceland Crystal created a puzzle regarding the physics of refraction that Huygens wanted to solve. Huygens eventually was able to solve this problem by means of elliptical waves in 1677 and confirmed his theory by experiments only after critical reactions in 1679.

His explanation was based on three hypotheses: There are inside the crystal two media in which light waves proceed,  one medium behaves as ordinary ether and carries the normally refracted ray, and the velocity of the waves in the other medium is dependent on direction, so that the waves do not expand in spherical form, but rather as ellipsoids of revolution; this second medium carries the abnormally refracted ray. By studying the symmetry of the crystal, Huygens was able to determine the direction of the axis of the ellipsoids, and from the refraction properties of the abnormal ray he established the proportion between the axes. He calculated the refraction of rays on plane sections of the crystal other than the natural crystal sides, and ultimately verified all his results experimentally.

Huygens intended to publish his results as part of the Dioptrica, but decided to separate his theory from the rest of the work at the last minute, marking the transition from geometrical optics to physical optics.