A History of Optics from Greek Antiquity to the Nineteenth Century by Darrigol Olivier

Author:Darrigol, Olivier
Language: eng
Format: epub
Publisher: OUP Oxford
Published: 2012-04-16T04:00:00+00:00

Crucial experiments

In 1803, Young gave his third Bakerian lecture, the second of those devoted to physical optics. His main new result was “an experimental demonstration of the general law of the interference of light.” Although “the ingenious and accurate Grimaldi” had long ago observed fringes in the shadow of a narrow rectangular blade, this phenomenon had generally been overlooked owing to Newton’s failure to notice it. Young not only studied these fringes with unprecedented accuracy, but he realized that they disappeared when the light passing on one side of the blade was blocked. In his opinion, this observation provided a direct proof of interference since it demonstrated the necessity of light traveling through two different paths. The Newtonian explanation by differential inflection near the edges of the blade failed, since the inflection on one side of the blade could not plausibly depend on the other side. In contrast, the law of interference immediately explained the equal spacing of the fringes in the plane of observation as well as their hyperbolic trajectory when the distance of this plane from the blade grew.48

Whereas Young had earlier justified interference by acoustic analogy, he now regarded the law of interference as establishing the analogy between light and sound:

For, hitherto, I have advanced in this Paper no general hypothesis whatever. But, since we know that sound diverges in concentric superficies, and that musical sounds consist of opposite qualities, capable of neutralising each other, and succeeding at certain equal intervals, which are different according to the difference of the note, we are fully authorised to conclude, that there must be some strong resemblance between the nature of sound and that of light.

Moreover, Young abandoned the Newtonian concept of inflection that he had used in his earlier accounts of diffraction. He now declared:

I have not, in the course of these investigations, found any reason to suppose the presence of such an inflecting medium in the neighbourhood of dense substances as I was formerly inclined to attribute to them; and, upon considering the phenomena of the aberration of the stars, I am disposed to believe, that the luminiferous ether pervades the substance of all material bodies with little or no resistance, as freely perhaps as the wind passes through a grove of trees.

According to this poetical metaphor, the ether could not accumulate in the vicinity of material bodies, so that inflecting atmospheres à la Mairan could not exist.49

The famous experiment of Young’s holes or slits appears in Young’s Lectures of 1807. It provides the simplest illustration of the principle of interference, since the two holes materially determine the paths of the interfering portions of light. It is also the optical counterpart of a case of water-wave interference described in the Lectures (Fig. 5.5). In conformity with his use of the phrase “undulatory theory of light,” Young now made water waves the central metaphor for any wave process:

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