Discovering The Nature Of Light: The Science And The Story by Fortson Norval;

Author:Fortson, Norval;
Language: eng
Format: epub
Publisher: World Scientific Publishing Company
Published: 2022-02-15T00:00:00+00:00

Fig. 4. (a) Newton’s Rings set up with a single (yellow) color source of incident light. (b) Reflection pattern, which consists of alternating yellow and dark rings. Contrast with Fig. 5 in Ch. 3.

Source: Optics and Lasers in Engineering, Volume 74, November 2015, pg 1–16.

Let us elaborate a bit more on this amazing thing; that Young could measure the wavelength of a color from the diameter of its rings! Suppose we pick a bright ring, which means that the bottom and top waves are inphase there. We should get another bright ring if we look further out where the extra distance that the bottom wave has traveled (i.e. twice the gap thickness) has increased by a complete wavelength λ. Since the change in gap thickness between the positions of the two adjacent rings was known, it told Young the value of λ. Young found that each color of light was associated with a definite value of λ, and the value of λ increased continuously as the prismatic color changed from blue to red. So, in this and other experiments, Young verified that wavelength is connected with color in the case of light, just as wavelength is connected with pitch in the case of sound in air, though in light the wavelength is much shorter.4

Young found that the length of a wave at the extreme red of the prismatic colors is equal to 665 nm (1 nm = 1 nanometer = 1 billionth of a meter) and at the extreme violet end is 420 nm.5 So light has very short wavelengths, the shortest by far of anything that had been measured to that time. Newton’s Rings gave the most accurate measurement of the wavelength of light for decades, and even in 1854, almost 200 years after the measurements were performed by Newton and 50 years after they were correctly interpreted by Young, they are quoted by Mary Somerville6 in her widely used textbook together with the following comment:

“The determination of these minute portions of space, which have a real existence, being the actual results of measurement, do as much honor to the genius of Newton as that of the law of gravitation.”7

These wavelengths determined by Young were of course for visible light. At about the same time, Sir William Hershel made a far-reaching discovery studying the spectrum of sunlight spread out by a prism. He placed the bulb of a thermometer at various locations in the spectrum and found that it read an elevated temperature at all locations. To Hershel’s surprise, the thermometer read an even higher temperature when it was moved past the red end of the spectrum where no light could be seen. He had discovered infrared (IR) radiation, invisible to the eye, but contained in the sunbeam. In subsequent experiments with a glass lens he found that this IR beam, detected by the thermometer, obeyed Snell’s Laws of reflection and refraction, but with an index of refraction in glass that was less than in the visible. Eventually, it

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