Black Body any object that is a perfect emitter and a perfect absorber of radiation >> object does not have to appear "black"

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1 Light is a... particle 1700's Newton Wave 1800's Young, Fresnel, Foucault, Maxwell, Hertz But Michelson and Morley 1887 failed to find the medium that light travels in in space Maxwell, in 1864, gave the medium argument an out but the implications of Michelson and Morley's NULL RESULT were instrumental in Einstein's Special Theory of Relativity And Then Planck 1900 Black body problem black bodies did not follow the wave model for the production of light when non luminous bodies were heated. (uv catastrophe) Black Body any object that is a perfect emitter and a perfect absorber of radiation >> object does not have to appear "black" sun and earth's surface behave approximately as black bodies the blacker the more ideal it becomes So, let's define some basic black body radiation laws spectrum Planck proposed On basis of empirical data (from observation or experiment), he developed a new formula which later showed remarkable agreement with accurate measurements of the spectrum of heat radiation. The result of this formula was so that energy is always emitted or absorbed in discrete units, which he called quanta. Planck developed his quantum theory further and derived a universal constant, which came to be known as Planck's constant. The resulting law states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant: E=fh, where h is 6.63 x Js Wein's Law can be used to predict the peak wavelength for a blackbody λmax = x 10 3 T where T is in Kelvin Questions page 597 and 598 1

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4 Problem #2 The Photoelectric Effect Einstein 1905 used Planck's idea of quanta and the notion of light as a particle a photon to explain the observed "PHOTOELECTRIC EFFECT" physics.suite101.com/article.cfm/the_photoelectric_effect ons/sims.php? sim=photoelectric_effect m/index.cfm? method=cuser.dsploginjoin Summary Observations colour of light, not the intensity, completed the circuit historic measured the energy of the moving electrons by creating a second voltage to shut off the electron flow cannot be explained with the wave model of light where energy is associated with amplitude Einstein's explanation light is a particle a photon which collides with the electrons in the atom. The momentum of the collision can transfer enough energy to the electron to free it from its atom. Once free, it will flow to the other side with any left over kinetic energy. The collision obeys the law of energy conservation Energy of the photon from Planck's equation ΣE before = ΣE after = energy to free the electron measured historically through the minimum energy required to free the electron it's threshold frequency + left over kinetic energy measured historically through a "cut off" voltage used to stop the flow of the electron hf = W + E k This equation is historically called the work function and is written based on its graphical origins E kin = h f W or y = mx = b fendt.de/ph14e/photoeffect.htm Note = energy units are J, ev where 1 ev = 1.6 x J textbook pg 600, hwk pg 628 #

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