Phys2212_38.1+to+38.7

Phys2212_38.1+to+38.7 - Physics 2212 Waves Lectur e 7...

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Unformatted text preview: Physics 2212 Waves Lectur e 7 Photoelectr ic Effect & Quanta 10/06/09 Physics 2212 - Lecture 7 2 Fraunhofer Absorption Lines Joseph von Fraunhofer (1787 - 1826) I n 1813, Joseph Fraunhofer, a glazier and telescope maker of Munich, discovered that there were dark lines in the spectrum of sunlight. He carefully described and catalogued some 574 lines and assigned the stronger ones letters A-K, designations that are still used by astronomers. The Fraunhofer lines are absorption lines caused by sunlight passing through the cooler and less dense gas surrounding the Sun, and provides a direct indication of the elemental composition of the Sun (and other stars). 10/06/09 Physics 2212 - Lecture 7 3 The Photoelectric Effect Heinrich Rudolph Hertz (1857 - 1894) I n 1887, in a corner of his physics classroom at the Karlsruhe Polytechnic University, Heinrich Hertz produced electromagnetic waves using a spark gap, and detected them with another loop and spark gap. He later observed that when ultraviolet (UV) light illuminated his spark gap he got better sparks and that UV illumination could also discharge an electroscope when charged negative (but not positive). He had discovered the photoelectr ic effect . J. J. Thomson suspected that the UV light was somehow causing metal surfaces to emit electrons, and demonstrated that the particles emitted had the same q/m as cathode rays. 10/06/09 Physics 2212 - Lecture 7 4 Characteristics of the Photoelectric Effect Phillip von Lenard (1862 - 1947) 1905 Nobel Prize Lenard was a student of Hertz, and in 1900 he investigated the photoelectric effect with the apparatus shown. He illuminated the cathode with UV and observed the cathode-to-anode current produced, in the presence of a retarding potential. He studied the current vs. wavelength and vs. retarding potential. He found the following: 1. The current I is proportional to light intensity; 2. The current I appears without delay as the light is turned on, even at low intensities; 3. There is no current unless frequency f > f ; 4. The value of f depends on the kind of metal of which the cathode is made; 5. When V is negative, current stops at V s . 6. The value of V s is independent of intensity of UV but depends on metal type. 10/06/09 Physics 2212 - Lecture 7 5 Classical Interpretation of the Photoelectric Effect From the point of view of classical physics, the discovery of the electromagnetic effect itself was not unexpected. I t was previously known that when metals were heated to around 1,500 o C, electrons were emitted. I llumination of a metal with UV light would be expected to strongly agitate the electrons while leaving the underlying structure of atoms relatively unperturbed. Thus, it should heat the electrons alone, causing their emission....
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Phys2212_38.1+to+38.7 - Physics 2212 Waves Lectur e 7...

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