Chapter34.pdf - The Photoelectric Effect Chapter 34 Lecture...

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Chapter 34 LectureQuantum PhysicsSlide 34-1The Photoelectric EffectThe photoelectric effect is the ejection of electrons from a metal surface when light shines on the surface.Classical physics predicts that– it should take some time before electronsit should take some time before electrons absorb enough energy to be ejected.the electrons’ kinetic energy should depend on the light intensitydepend on the light intensity.the wavelength of the light shouldn’t matter.Experiment shows thatelectrons are ejected immediately.– electron energy is independent of lightelectron energy is independent of light intensity.electron kinetic energy depends on wavelength with no electrons ejected forSlide 34-2wavelength, with no electrons ejected for wavelengths longer than some maximum value that depends on the metal.Einstein’s ResolutionIn 1905, the same year he developed special relativity, Einstein offered an explanation of the photoelectric effect.Einstein suggested that light energy comes in particle-like “bundles” called quanta or photons.The energy of a single photon is given by E = hf, with fthe frequency of the light and h= 6.626 x 10-34J∙s is Planck’s constant.The more intense the light the more photonsbut the energy ofThe more intense the light, the more photons – but the energy of each photon is unrelated to the light intensity.Each material has a minimum energyll d thk ftiid tcalled the work function, , required to eject an electron.Each photon can give all its energy to an electron in the metalThe electrons emerge with maximum kinetic energy given bySlide 34-3maxKhfEinstein’s InterpretationThe maximum KE, depends max,Khfonly on the frequency and the work function, not on the light intensity.The maximum KE increases with increasing maxfrequency.The effect is instantaneous since there is a one-to-one interaction between the photon d thltand the electron.The effect is not observed below a certain cutoff frequency fcsince the photon energy must be greater than or equal to the workmust be greater than or equal to the work function:The corresponding cutoff wavelength iscfhThe corresponding cutoff wavelength is given by:The figure shows the linear relationship betweenKand ƒ1,240 eVnmchcSlide 34-4between Kmaxand ƒThe slope of each line is hccf
Clicker QuestionWhat is the approximate energy of a photon of red lightWhat is the approximate energy of a photon of red light (=635 nm)?A.0.5 eVB. 1.0 eVB.1.0 eVC.2.0 eVD3 0 eVD.3.0 eVSlide 34-5Clicker QuestionA red and green laser both produce light at a power level of 5 mW. Which one produces more photons/second?A. RedB. GreenC. SameSlide 34-6Clicker Question

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