If you want to know your progress so far, please send me an email request [email protected] ReviewReminder (for those who don’t read syllabus)A photon (quantum of light) is reflected from a mirror. True or false: (a) Because a photon has a zero mass, it does not exeProblem: a lightbulb27.2 Photoelectric EffectPhotoelectric Effect SchematicPhotoelectric Current/Voltage GraphFeatures Not Explained by Classical Physics/Wave TheoryEinstein’s ExplanationExplanation of Classical “Problems”Verification of Einstein’s TheoryProblem: photoeffect27.3 Application: Photocells27.4 X-RaysProduction of X-raysProduction of X-raysDiffraction of X-rays by CrystalsSchematic for X-ray DiffractionPhoto of X-ray Diffraction PatternBragg’s LawIf you want to know your progress so far, please send me an email request [email protected]/10/0311/10/03General Physics (PHY 2140)Lecture 28Lecture 28¾ Modern Physics9Quantum Physics9Photoelectric effectChapter 27http://www.physics.wayne.edu/~apetrov/PHY2140/2211/10/0311/10/03If you want to know If you want to know your progressyour progressso far, please so far, please send me send me an email requestan email requestatatapetrovapetrov@[email protected]/10/0311/10/03Lightning ReviewLightning ReviewLast lecture:1.1.Quantum physicsQuantum physics99Blackbody radiationBlackbody radiation99Planck’s hypothesis2max0.2898 10TmKλ−=×⋅Planck’s hypothesis,1,2,3,...nEnhf n==Review Problem:All objects radiate energy. Why, then, are we not able to see all objects in a dark room?4411/10/0311/10/03Reminder (for those who don’t read Reminder (for those who don’t read syllabussyllabus))Reading Quizzes (bonus 5%):It is important for you to come to class prepared, i.e. be familiar with the material to be presented. To test your preparedness, a simple five-minute quiz, testing your qualitative familiarity with the material to be discussed in class, will be given at the beginning of some of the classes. No make-up reading quizzes will be given.There could be one today…… but then again…5511/10/0311/10/03QUICK QUIZA photon (quantum of light) is reflected from a mirror. A photon (quantum of light) is reflected from a mirror. True or falseTrue or false: : (a) Because a photon has a zero mass, it does not exert a force (a) Because a photon has a zero mass, it does not exert a force on on the mirror. the mirror. (b) Although the photon has energy, it cannot transfer any energ(b) Although the photon has energy, it cannot transfer any energy to y to the surface because it has zero mass. the surface because it has zero mass. (c) The photon carries momentum, and when it reflects off the mi(c) The photon carries momentum, and when it reflects off the mirror, rror, it undergoes a change in momentum and exerts a force on it undergoes a change in momentum and exerts a force on the mirror. the mirror. (d) Although the photon carries momentum, its change in momentum(d) Although the photon carries momentum, its change in momentumis zero when it reflects from the mirror, so it cannot exert a is zero when it reflects from the mirror, so it cannot exert a force on the mirror.force on the mirror.(a)(a)False False (b)(b)False False (c)(c)True True (d)(d)FalsepFt∆=False6611/10/0311/10/03Problem: a Problem: a lightbulblightbulbAssuming that the tungsten filament of a Assuming that the tungsten filament of a lightbulblightbulbis a blackbody, is a blackbody, determine its peak wavelength if its temperature is 2 900 K.determine its peak wavelength if its temperature is 2 900 K.7711/10/0311/10/03Assuming that the tungsten filament of a Assuming that the tungsten filament of a lightbulblightbulbis a blackbody, determine its is a blackbody, determine its peak wavelength if its temperature is 2 900 K.peak wavelength if its temperature is 2 900 K.Given:T = 2900 K2900 KFind:λmax= ?Recall Wein;s law: λλmaxmaxT = 0.2898 x 10T = 0.2898 x 10--22m • K.m • K. It can be used to solve for λmax:2max0.2898 10mKTλ−×⋅=Thus, ()27max0.2898 109.99 10 9992900mKmnmKλ−−×⋅==×=This is infrared, so most of the electric energy goes into heat!8811/10/0311/10/0327.2 Photoelectric Effect27.2 Photoelectric EffectWhen light is incident on certain metallic surfaces, electrons aWhen light is incident on certain metallic surfaces, electrons are re emitted from the surfaceemitted from the surfaceThis is called the This is called the photoelectric effectphotoelectric effectThe emitted electrons are called The emitted electrons are called photoelectronsphotoelectronsThe effect was first discovered by HertzThe effect was first discovered by HertzThe successful explanation of the effect was given by Einstein iThe successful explanation of the effect was given by Einstein in n 19051905Received Nobel Prize in 1921 for paper on electromagnetic radiatReceived Nobel Prize in 1921 for paper on electromagnetic radiation, of ion, of which the photoelectric effect was a partwhich the photoelectric effect was a part9911/10/0311/10/03Photoelectric Effect SchematicPhotoelectric Effect SchematicWhen light strikes E, When light strikes E, photoelectrons are emittedphotoelectrons are emittedElectrons collected at C and Electrons collected at C and passing through the ammeter passing through the ammeter are a current in the circuitare a current in the circuitC is maintained at a positive C is maintained at a positive potential by the power supplypotential by the power supply101011/10/0311/10/03Photoelectric Current/Voltage GraphPhotoelectric Current/Voltage GraphThe current increases with The current increases with intensity, but reaches a intensity, but reaches a saturation level for large saturation level for large ∆∆VV’’ssNo current flows for voltages No current flows for voltages less than or equal to less than or equal to ––∆∆VVss, the , the stopping potentialstopping potentialThe stopping potential is The stopping potential is independent of the radiation independent of the radiation intensityintensity111111/10/0311/10/03Features Not Explained by Classical Features Not Explained by Classical Physics/Wave TheoryPhysics/Wave TheoryNo electrons are emitted if the incident light frequency is No electrons are emitted if the incident light frequency is below some below some cutoff frequencycutoff frequencythat is characteristic of the that is characteristic of the material being illuminatedmaterial being