Photoelectric Equation:

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# Photoelectric Equation: - PowerPoint PPT Presentation

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 10-19 J?

Photoelectric Equation:

W = (6.63 x 10-34 Js)(1.09 x 1015 Hz) =7.20 x 10-19 J

Total Energy, ETotal Relativistic Energy

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:

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

Momentum from K:Finding Momentum from K.E.

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 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

Planck’s Equation:

E = hf (h = 6.626 x 10-34 J s)

Photon

1 eV = 1.60 x 10-19 J

The Electron-volt:

E = hf

1 MeV = 1.6 x 10-13 J

1 keV = 1.6 x 10-16 J

Summary

Apparently, light consists of tiny bundles of energy called photons, each having a well-defined quantum of energy.

Incident light

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

Stopping potential

V

Slope

y

fo

x

Frequency

Summary (Cont.)

Planck’s Experiment:

Incident light

Cathode

Anode

V

A

+

-

Potentiometer

Kmax = eVo

Wavelength of a photon:

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