Incident light. Cathode. Anode. A. V. +. . Stopping potential:. V o = 0.800 V. Example 4: The threshold frequency for a given surface is 1.09 x 10 15 Hz . What is the stopping potential for incident light whose photon energy is 8.48 x 10 19 J ? . Photoelectric Equation:.
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Cathode
Anode
A
V
+

Stopping potential:
Vo= 0.800 V
Example 4:The threshold frequency for a given surface is 1.09 x 1015 Hz. What is the stopping potential for incident light whose photon energy is 8.48 x 1019 J?
Photoelectric Equation:
W = (6.63 x 1034 Js)(1.09 x 1015 Hz) =7.20 x 1019 J
Recall that the formula for the relativistic total energy was given by:
For a particle with zero momentum p= 0:
E =moc2
A light photon has mo= 0, but it does have momentum p:
E =pc
de Broglie Wavelength:
Waves and ParticlesWe know that light behaves as both a wave and a particle. The rest mass of a photon is zero, and its wavelength can be found from momentum.
All objects, not just EM waves, have wavelengths which can be found from their momentum
In working with particles of momentum p = mv, it is often necessary to find the momentum from the given kinetic energy K. Recall the formulas:
K = ½mv2 ; p = mv
Multiply first Equation by m:
mK =½m2v2= ½p2
e
90 eV
Example 5:What is the de Broglie wavelength of a 90eV electron? (me = 9.1 x 1031 kg.)Next, we find momentum from the kinetic energy:
p = 5.12x 1024 kg m/s
l = 0.122 nm
E = hf (h = 6.626 x 1034 J s)
Photon
1 eV = 1.60 x 1019 J
The Electronvolt:
E = hf
1 MeV = 1.6 x 1013 J
1 keV = 1.6 x 1016 J
SummaryApparently, light consists of tiny bundles of energy called photons, each having a welldefined quantum of energy.
Cathode
Anode
A
C
Ammeter
Threshold wavelength lo
A
+

Summary (Cont.)If lis innm, the energy ineV is found from:
Wavelength in nm; Energy in eV
V
Slope
y
fo
x
Frequency
Summary (Cont.)Planck’s Experiment:
Incident light
Cathode
Anode
V
A
+

Potentiometer
Kmax = eVo
de Broglie Wavelength:
Summary (Cont.)Quantum physics works for waves or particles:
For a particle with zero momentump = 0:
E =moc2
A light photon has mo = 0, but it does have momentum p:
E =pc