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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Incident light
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
Total Energy, E
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
Wavelength of a photon:
de Broglie Wavelength:
We 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
Momentum from K:
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
Next, we find momentum from the kinetic energy:
p = 5.12x 1024 kg m/s
l = 0.122 nm
Planck’s Equation:
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
Apparently, light consists of tiny bundles of energy called photons, each having a welldefined quantum of energy.
Incident light
Cathode
Anode
A
C
Ammeter
Threshold wavelength lo
A
+

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

Potentiometer
Kmax = eVo
Wavelength of a photon:
de Broglie Wavelength:
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