photo electric effect

1. Photoelectric EffectPhotoelectric Effect

2. AbsorptionAbsorption
 Charges can absorb electromagnetic waves.Charges can absorb electromagnetic waves.
• Can cause charge to accelerateCan cause charge to accelerate
Moving chargeabsorbed photon

3. Ejected ElectronsEjected Electrons
 Electromagnetic energyElectromagnetic energy
could eject electrons fromcould eject electrons from
the surface.the surface.
• Accelerate with voltageAccelerate with voltage
• Measure currentMeasure current
 Classical expectation is thatClassical expectation is that
the current varies withthe current varies with
intensity.intensity.
• May also vary withMay also vary with
frequency.frequency. intensity∝I
V
e
I

4. Stopping PotentialStopping Potential
 As the voltage is decreasedAs the voltage is decreased
the photocurrent decreases.the photocurrent decreases.
• Matches classical viewMatches classical view
 The voltage can be reversedThe voltage can be reversed
and some current flows.and some current flows.
• Stopping point independentStopping point independent
of intensityof intensity
• Characteristic of metalCharacteristic of metal
eVKE =max
V
e
I

5. Atomic ViewAtomic View
 A photon can eject an electronA photon can eject an electron
from an atom.from an atom.
• Photon absorbedPhoton absorbed
• Electron releasedElectron released
 Energy within the atom isEnergy within the atom is
quantized.quantized.
• Minimum energyMinimum energy φφ needed forneeded for
interactioninteraction
• Einstein’s photoelectricEinstein’s photoelectric
equationequation
e
γ
Z
φ−= hfeVS

6. Work FunctionWork Function
 The minimum energy is theThe minimum energy is the
work function of the metal.work function of the metal.
 Below a certain frequency noBelow a certain frequency no
current can flow.current can flow.
• ff00 == φφ/h/h
 Work functions for someWork functions for some
metals:metals:
• Na: 2.28 eVNa: 2.28 eV
• Co: 3.90 eVCo: 3.90 eV
• Al: 4.08 eVAl: 4.08 eV
• Pb: 4.14 eVPb: 4.14 eV
• Zn: 4.31 eVZn: 4.31 eV
• Fe: 4.50 eVFe: 4.50 eV
• Cu: 4.70 eVCu: 4.70 eV
• Ag: 4.73 eVAg: 4.73 eV
• Pt: 6.35 eVPt: 6.35 eV

7. PhotocellPhotocell
 The photoelectric effect is commonly used to measureThe photoelectric effect is commonly used to measure
light.light.
• Camera light meterCamera light meter
 It can also generate electricity.It can also generate electricity.
• Photovoltaic cellPhotovoltaic cell
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