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:.
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 10-19 J?
W = (6.63 x 10-34 Js)(1.09 x 1015 Hz) =7.20 x 10-19 J
Recall that the formula for the relativistic total energy was given by:
For a particle with zero momentum p= 0:
A light photon has mo= 0, but it does have momentum p:
de Broglie Wavelength:Waves and Particles
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
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
90 eVExample 5:What is the de Broglie wavelength of a 90-eV electron? (me = 9.1 x 10-31 kg.)
Next, we find momentum from the kinetic energy:
p = 5.12x 10-24 kg m/s
l = 0.122 nm
E = hf (h = 6.626 x 10-34 J s)
1 eV = 1.60 x 10-19 J
E = hf
1 MeV = 1.6 x 10-13 J
1 keV = 1.6 x 10-16 JSummary
Apparently, light consists of tiny bundles of energy called photons, each having a well-defined quantum of energy.
Threshold wavelength lo
If lis innm, the energy ineV is found from:
Wavelength in nm; Energy in eV
Kmax = eVo
de Broglie Wavelength:Summary (Cont.)
Quantum physics works for waves or particles:
For a particle with zero momentump = 0:
A light photon has mo = 0, but it does have momentum p: