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The Particle of Light

• A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms.

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The Photoelectric Effect

• Many physicists’ work contributed to the discovery of the photoelectric effect

• What is it?• The ability of light to dislodge electrons from a

metallic surface• The electrons can be detected and the resulting

signals amplified• Lots of applications in visual imaging

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Questions

• How many electrons are ejected in a given time?• How does this number depend of wavelength or

intensity?• How energetic of the ejected electrons?• Upon what does the electron energy depend?• Are electrons ejected instantly or is there a time

delay?

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Photoelectric Experiments

• Cathode – electrons are ejected• Anode – electrons are collected

Experiment 1 Experiment 2

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Photoelectric Experiments - con’t

• a) Electrons freely flow from the anode back to the cathode and they are counted along the way• Can determine how # of e- depends on

wavelength and intensity; time light must shine on cathode for electrons to flow

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Photoelectric Experiments - con’t

• b) Ejected electrons have to overcome the electric field to get to the anode

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Photoelectric Experiments - con’t• b) Ejected electrons have to overcome the electric

field to get to the anode• Can determine energy of ejected electron• If the potential difference between the plates,

ΔΦ = 2.0 V, the difference between the electron’s electrostatic potential energy at the anode and its potential energy at the cathode is

• The electron can make it to the anode only if it has an initial kinetic energy greater than this

€

qΔφ = 1.6 ×10−19C( ) 2.0V( ) = 3.2 ×10−19J

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Wave Model Predictions• The rate at which electrons are ejected from a metal is

proportional to the intensity of the incident light.

• Lower intensity light rays should have a delay before electrons are ejected

• The rate may depend on frequency (wavelength) of light

• The maximum kinetic energy of the electrons is likely to increase with increasing intensity

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Experiments Provide the Following Results• At high intensities and fixed frequencies, the # of

ejected electrons is proportional to intensity• Electrons are ejected instantly, regardless of

intensity level• For constant intensity, the # of electrons decreases

with increasing frequency• If the frequency is below a certain level, no electrons

are ejected, regardless of intensity level• Above the cutoff frequency, the electrons’ maximum

kinetic energy is propostional to the frequency of light

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Maximum Energy depends on Frequency

• Above the cutoff frequency, the electrons’ maximum kinetic energy is proportional to the frequency of light

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Einstein’s Prediction – light is a particle

• Light consists of particles, each carrying a certain amount of energy

• Where E is the energy, f is the frequency, and h is Planck’s constant

• We typically express colors of light in wavelengths€

E = hf = hcλ

€

h = 6.63×10−34 J ⋅s = 4.15 ×10−15eV ⋅ s

€

f = cλ

€

hc =1240eV ⋅nm

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Einstein’s Prediction - con’t

• Einstein also predicted that each electron ejected from the metal was a result of a collision with a single photon

• Where K is the kinetic energy of the electron and W is the work function for the metal• The work function is the energy required to liberate the

electron from the metal

€

K = hcλ−W

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Einstein’s Prediction - con’t

• Einstein’s model explains the experimental results so neatly, why was there resistance in the science community?• This model is completely inconsistent with the wave nature

of light.

• Neither model, wave or particle, adequately explains light by itself

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Extra Credit

• Several extra credit projects have been added to the schedule.

• Check Moodle and the online schedule for more info.

• Acousto-magnetic strips deter shoplifters, due 4/23• Blue Man Group Pipe Instruments, due 4/23• Make an instrument, due 4/28• Applications of the photoelectric effect, due 4/28

Practice• Interactive activity – photoelectric effect for

different metals• Go to the site below and answer the questions at the

bottom of the page• http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo

.htm• Interactive problem

• http://wug.physics.uiuc.edu/cgi/courses/shell/per/phys102/ie.pl?12/pe1

• Group Problems• Q3B.5, Q3S.4

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