Thomas Young and the Wave Nature of Light Historical Background Isaac Newton was famous not only for formulating the laws of motion but also for pioneering in the study of optics. He used a prism to show that sunlight was a mixture of the colors that make up the rainbow. In his Opticks (1704), Newton argued that light was made up of tiny particles. Slightly earlier, the Dutch physicist Christiaan Huygens wrote a Treatise on light, in which he proposed that light was a wave. It was only in 1789 that Thomas Young proposed a simple experiment that appeared to resolve the controversy by showing that light indeed behaves as a wave (according to 20th-century quantum mechanics, however, even ...view middle of the document...
When the hair approached so near to the direction of the margin of a candle that the inflected light was sufficiently copious to produce a sensible effect, the fringes [alternating bands] began to appear; and it was easy to estimate the proportion of their breadth to the apparent breadth of the hair across the image of which they extended. I found that six of the brightest red fringes, nearly at equal distance, occupied the whole of that image. The breadth of the aperture was 66/1000 [of an inch], and its distance from the hair 8/10 of an inch; the diameter of the hair was ... 1/600 [of an inch]. Hence, we have 11/1000 for the deviation of the first red fringe at the distance of 8/10; and as 8/10 / 11/1000 = 1/600 / 11/480000, or 1/43636 [of an inch] for the difference of the routes of the red light where it was most intense.”
Young has thus measured the wavelength of candle light to be 1/43,636 of an inch, or about 582 nm [1 nanometer = 10-9 m; present measurements give yellow light a wavelength ranging from 550 to 600 nm]. Young provided no diagrams nor did he discuss the geometry upon which his computations rested, but he did provide a verbal explanation of the idea that underlies the calculation [ibid., 62]:
... wherever two portions of the same light arrive at the eye by different routes, either exactly or very nearly in the same direction, the light becomes more intense when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions; and this length is different for light of different colors.... [that is, different colored lights have different wavelengths]
The wave analogy is a powerful idea with many applications throughout science and society. All waves (including sound, light, radar and x-rays) and even some things that are usually thought of as particles (like electrons!) show behavior that can be explained using the wave analogy and the concept of interference.
Thomas Young and the Wave Nature of Light Revised 4/19/06 by DSA for P1 p. 1/8
How waves interfere with each other Young took the alternating light and dark band pattern he observed to be evidence of light behaving like a wave. In order to understand how Young concluded this, we need to examine how waves interact with each other and with barriers. Suppose you are lying on the beach and you observe an ocean wave passing through a small break in a jetty. On the ocean side of the jetty, the crests form lines parallel to the shore. On the inland side of the jetty, the crests form semicircles (see photo at right). If there are two such openings near each other, two circular waves are created and the two waves interact with each other. A diagram from Thomas Young’s work is shown below. The two openings, labeled A and B, are on the left side of the diagram. The circles represent the crests of the waves.
Plane waves pass through a small opening in a barrier, resulting in circular waves....