astllas - Laboratory 8 - Properties of Light and Atomic...

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54 Laboratory 8 - Properties of Light and Atomic Spectra Materials Used: Spectrometers, CLEA software 1 . Objectives: To explore the fundamental nature of light; to become acquainted with spectroscopy . Discussion: From the time of Sir Isaac Newton (1642 - 1727) to the beginning of the 20th century the fundamental nature of light was a topic of debate among scientists. Newton proposed in his book Opticks (1704) that light consisted of a stream of particles know as corpuscles . With this theory Newton was able to explain phenomena such as reflection and refraction. Newton’s corpuscular theory was widely accepted both because it seemed to explain known phenomena and because Newton was its architect. A contemporary of Newton, Dutch physicist and astronomer Christian Huygens 2 (1629 - 1695), proposed an alternative wave theory that not only explained reflection and refraction (more elegantly than Newton’s corpuscular theory) but also diffraction - the bending of light as it passes around sharp edges or through narrow openings. In spite of its success, Huygen's theory did not receive wide acceptance. All waves known at the time traveled through a physical medium of some sort (air, water, etc.). Light, on the other hand, was known to travel through the vacuum of space – something that seemed inconsistent with the requirements of known waves. In 1801 Thomas Young conducted an experiment that demonstrated another phenomena, interference , which could only be explained by the wave theory of light. Interference occurs when light waves are combined. Although there are many possibilities from such an event Young demonstrated that cancellation of light waves was possible by combining them in just the right way. Figure 1 depicts both constructive and destructive interference. In constructive interference identical light waves that are in phase (i.e., the crests and troughs of each wave line up) add in such a manner that the combined wave has an amplitude (A) that is twice that of either individual wave. In destructive interference waves that are exactly out of phase (i.e., the crest of one wave corresponds to the trough of the other) cancel when combined. Such behavior could not be explained by the corpuscular theory since no mechanism known at the time could explain how two particles could combine so as to cancel. Half a century later Maxwell, Hertz and others demonstrated definitively that light was a high frequency electromagnetic wave (like radio or television waves) that traveled through free space with a speed of 3 x 10 8 m/s. By the dawn of the 20th century, the wave theory of light was widely accepted. 1 The computerized portions of this lab were provided courtesy of Gettysburg College. 2

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This note was uploaded on 01/16/2012 for the course PHYS 153 taught by Professor Hackwrth,m during the Spring '08 term at Idaho State University.

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astllas - Laboratory 8 - Properties of Light and Atomic...

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