THE PHOTOELECTRIC EFFECT
 powerful evidence that light is particulate (behaves like little bullets)
 not explained by a wave theory
 When light in the form of photons (packets of discrete electromagnetic wave
energy) falls on a metal surface, electrons are ejected from the surface.
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All the photon energy is given up to one electron
Photon energy = hf
Emission is instantaneous
Excess energy over and above that required for ejection is picked up as kinetic
energy of the moving electron
Einstein’s Photoelectric Equation
incident photon energy = energy required to overcome electrostatic attraction of
surrounding nuclei (WORK FUNCTION , ) + maximum kinetic energy of the emitted
electron.
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usually measured in eV (1eV = 1.6 x 10-1J)
hf = + 1/2mv2max
 If f < a threshold frequency, fo no emission occurs no matter how bright the
illumination (i e, more photons)
The kinetic energy (speed) of the emitted electrons is variable. This is because
electrons deeper in the lattice require more energy to get them out ( higher  than
those immediately at the surface.
Emission depends on photon energy, (or frequency E=hf), NOT on intensity
A graph of Ek against incident frequency is a straight line, intersecting the x axis at the
THRESHOLD frequency, fo below which no emission can occur. The slope of this graph
is h, the y-intercept is 
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THE PHOTOELECTRIC EFFECT
Ek
f
When a positive potential is applied to the collector, which collects the emitted
photoelectrons, a fraction of the total number emitted is collected. If the potential
rises sufficiently, all the emitted electrons are collected.
I
Bright light
Graphically:
Dimmer light
V
Vs
If a negative potential, Vs called the STOPPING POTENTIAL is applied, the
photoelectric current will decrease to zero, for a given photon energy
Decide with a reason, which metal has the higher work function by looking at the
graph below. The two metals have been illuminated identically.
I
V
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THE PHOTOELECTRIC EFFECT
Problems.
Check this applet out.
http://physics.berea.edu/%7Eking/Teaching/ModPhys/QM/Photoelectric/Photoelectri
c.html
A particular metal has  = 2.4 eV. What is the maximum wavelength of illumination
required to eject an electron from its surface (recall c = fWhere is this light in the
EM spectrum?
Light of frequency 3 x 1015Hz falls on the surface. Find the max speed of the emitted
electrons, also their energy in eV. (mass of an electron = 9x10-31 kg)
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