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Photoelectric effect, photons

Photoelectric effect, photons. Physics 123. Concepts. Photoelectric effect Work function and stopping potential ElectronVolt (eV) Photons Blackbody radiation. Photoelectric effect. Light shines on metal surface P Electrons are emitted from P Current flows between P and C

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Photoelectric effect, photons

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  1. Photoelectric effect, photons Physics 123 Lecture X

  2. Concepts • Photoelectric effect • Work function and stopping potential • ElectronVolt (eV) • Photons • Blackbody radiation Lecture X

  3. Photoelectric effect • Light shines on metal surface P • Electrons are emitted from P • Current flows between P and C • Measure kinetic energy of emitted electrons KEeby applying a stoppingpotential V0: KEe=eV0 • No surprises so far, because light as a wave carries energy Lecture X

  4. Electronvolt • Energy that one electron gains when being accelerated over 1V potential difference is called electronvolt eV: • 1eV=1.6x10-19C 1V= 1.6x10-19J • Yet another unit to measure energy, • Commonly used in atomic and particle physics. Lecture X

  5. Photoelectric effect light W0 e- e- • Inside metal electrons are sitting in potential “wells” • Need supply some minimum energy W0 to get them out • W0is called work function – different for different materials • Light provides this energy EL EL=Ne(W0+KEe) • Ne- number of emitted electrons Lecture X

  6. Photoelectric effect EL=Ne(W0+KEe) • Here is a surprise: • Increase light intensity EL– expect • More electrons Ne - True • With higher kinetic energyKEe – False • Do not expect KE to depend on light wavelength l , but it does • More over if l>l0– no electrons come out no matter how intensive the light source is!!! Lecture X

  7. Photoelectric effect W0 e- e- • Explanation of photoeffect A.Einstein • Postulate: light is transmitted in tiny particles (!!) – photons (g) • Each photon carries energy proportional to its frequency: Eg=hf=hc/l • Planck’s constant h=6.626 x 10-34 J.s • hc=1243 eV.nm • One electron absorbs only one photon • If Eg<W0– no electrons are emitted: • Eg<W0 f<f0l>l0 Eg Eg Eg f0=W0/h l0=hc/W0 Lecture X

  8. Photoelectric effect W0 e- e- Eg=hf=hc/l • Photon energy (Eg) is spent to get electron out (W0) and on electron’s kinetic energy (KEe): Eg= W0+ KEe • If stopping potential is applied electron’s kinetic energy KEeis converted into potential energy of the electron eV0: Eg= W0+ eV0 Eg Eg Eg KEe Lecture X

  9. Photons – particles of light • Photon= particle of light = quantum of light = gamma(g)-quant • Intensity of electromagnetic wave – sum of energies carried by quanta of light – photons: I=NEg • Each photon carries energy proportional to the frequency of the EM wave: Eg=hf=hc/l Lecture X

  10. Problem 38-15 • I=0 when l>l0=570nm • Work function W0-? • What stopping potential (V0) must be applied if light ofl=400 nm is used? Lecture X

  11. Photon absorption and emission • Photons can be absorbed by matter – converted into other forms of energy – e.g. kinetic, potential, thermal. At this point photon ceases to exist Final energy=Initial energy+Eg • Photons can be emitted thus reducing the energy of the remaining system. Photon was NOT hiding inside matter waiting to be released, it is created by converting other forms of energy into light (EM-wave) Final energy+Eg=Initial energy • Energy is conserved in either case Lecture X

  12. More particle properties of light W0 e- e- • Sun tan – • chemical reaction with threshold energy W0 • visible light l=700-400 nm does not have enough energy to start this reaction • ultraviolet – l<400 nm - more energetic light – does have enough energy • Photographic film exposure • Why red light is safe? Eg Eg Eg KEe Lecture X

  13. Blackbody radiation • Another problem was elegantly solved by introducing “light particles” • Classical explanation of Blackbody (no reflection radiation diverged at low wavelength • Max Planck suggested replacing integral over frequencies with summing a series (photons!) peak lpT=2.9x10-3 mK • Planck did not seek deep meaning behind this seemingly mathematical trick • Fundamental explanation was suggested by Einstein in 1905 – energy is quantized Lecture X

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