James+Trapp

=Thomas Young (1773 - 1829) =

The Double Slit Experiment


Biography: **Thomas Young** was born in Milverton, Somersetshire, England in the year 1773. Thomas Young was an Egyptologist who helped decipher the Rosetta Stone which was important in the reading of Egyptian hieroglyphics by decoding it through Greek translations. Thomas Young received a degree in medicine at the University of Göttingen in 1796. Thomas Young was also a physicist who had a career in serving in public office. Young served as foreign secretary to the Royal Society from 1802 to his death in 1829 and was apart of several other scientific commissions. Thomas Young was also secretary to England’s Board of Longitude after 1818 and was the Nautical Almanac’s editor. Thomas Young’s accomplishments include the first description and measurement of astigmatism, the first psychological explanation for color sensation, and work on the theories of capillary action and elasticity. Thomas Young has written several writings on the subjects of Egyptology, medicine, and physics.

The Double Slit Experiment: Thomas Young’s notable accomplishment in quantum physics is his “double-slit experiment” in 1803 which supported the wave theory of light. The wave theory of light was not popular at the time and the particle theory of light was accepted. This debate stems back to the late 1600s when Sir Isaac Newton and Christiaan Huygens both had diverging theories to whether light was a wave, as Huygens theorized, or a particle, as Newton theorized. The wave theory could not explain everything about light such as optical polarization because only waves that moved parallel to the wave travel’s direction were known at the time. Newton could not explain the interference of light and eventually included wavelike properties to his theory. The shortcomings of Huygens’s theory and Newton’s notoriety is what led people to believe the particle theory of light more than the wave theory of light. This favor of the particle theory would continue from the late 1600’s into the 1800’s. __** Here is a diagram of what his experiment looked like: **__

 The “double-slit” experiment involves shining a light through a hole in one screen and two slits on another screen. From each screen, two sets of light waves spread out and interfere with each other and it results in a pattern of light stripes (constructive interference) and dark stripes (destructive interference) which are called interference fringes. You can measure the light’s wavelength by measuring the spacing of the fringes. The brightest point is always at the midpoint between the two slits where the two waves interfere constructively. The experiment officially established support for Huygens’s wave theory of light because of the interference that occurred. As Thomas Young explained in 1807, “the middle of the pattern is always light, and the bright stripes on each side are at such distances that the light coming to them from one of the apertures must have passed through a longer space than that which comes from the other by an interval which is equal to the breadth of one, two, three, or more of the supposed undulations [wavelengths], while the intervening dark spaces correspond to a difference of half a supposed undulation, of one and a half, of two and a half, or more.” In other words, the stripes spread out evenly across the pattern in specific intervals according to the wavelength. Hence you can measure the wavelength of light based on the spaces between the interference fringes. __** Here is a simpler diagram: **__  How did it affect Science?: This experiment would lay the groundwork to the eventual merging of Huygens’s and Newton’s theories which lead to the modern quantum theories developed around the start of the 20th century. At first, the particle theory was still the popular theory for what light was. However, in 1815, French physicist Augustin Fresnel supported Young’s findings with calculations and even predicted new properties. The two theories continued to merge when Irish mathematician Sir William Hamilton developed a theory of unifying optics and mechanics. The development of more modern theories of quantum physics would come as physicists Max Planck, who discovered blackbody radiation, and Albert Einstein, who discovered the photoelectric effect, discovered particle-like properties to light. The particles are called light quanta, or as they are called now, photons. The wave-particle duality of light would be demonstrated again in 1987 when Japanese scientists repeated Young’s experiment with electrons. The electrons left the electron “gun” and arrived on the detection screen like a particle but it interfered like waves would as shown in the original experiment with light. The experiment has also been established as a way to explain quantum physics in science classes in school because of how simple it is to demonstrate. Ultimately, Young’s experiment helped bring credibility to the wave property of light and helped develop the modern conception of the wave-particle duality of light. If it weren’t for Thomas Young, the particle theory of light would have continued to be the explanation for light for much longer than it was and may have been the modern theory of what light really is. Report by James Trapp and Matt Knoecklein

Works Cited:
 * http://sciencecity.oupchina.com.hk/npaw/student/glossary/youngs_double_slit.htm
 * http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/
 * [|http://encarta.msn.com]
 *  Gribbin, John. __The Experiment with Two Holes__.__Encarta__. CD-ROM. Microsoft, 2003.
 *  Marburger, John H. __Light__.__Encarta__. CD-ROM Microsoft, 2003.
 *  McGrath, Kimberley A., ed. "Double-Slit Interference Experiment." __World of Physics__. 2 vols. Detroit: Gale group, 2001. 176-77.
 *  __Optics__.__Encarta__. CD-ROM. Microsoft, 2003.
 *  __Young, Thomas__.__Encarta__. CD-ROM. Microsoft, 2003.