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PHOTOELECTIC EFFECT
PARTICLE MODEL OF LIGHT
PHOTOELECTRIC EFFECT
EMR falling on a metal surface can eject electrons (photocurrent)
Hert’z experiments – speed of radio waves (same as light)
1886-7 Hertz’s observation of the effect of a radio wave on a receiver – photoelectric
effect - UV can cause electrons to be emitted from a metal surface (failed to investigate)
Experimental results could not be explained by classical wave theory for light
 KE of ejected electrons depends on frequency not intensity
 Number of emitted electrons depends on intensity
 If frequency f of EMR less than some threshold – no electrons ejected
 Electrons emitted immediately when light falls on metal surface
Application of photoelectric effect
light meters (camera); photocells; photoconductors; light detectors
light – photons E = h f
eG galvanometer
– measures small
currents
I
G
Vs stopping voltage
varied  I = 0
qe Vs = ½ m vmax2
Do problems
Vs
p2.05
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EINSTEIN’S QUANTUM EXPLANATION OF PHOTOELECTRIC
EFFECT / QUANTIZATION OF ENERGY – PARTICLE MODEL OF
LIGHT
Albert Einstein (1905) Photoelectric Effect - contribution to quantum theory
particle nature of light (electromagnetic radiation) – photons
energy of a photon E = h f
Einstein’s idea of light as a stream of particles called photons with energy E = h f came
from Planck’s work on blackbody radiation.
h f  W  12 m v 2
Einstein’s explanation
h f  Wmin  12 m vmax 2  Wmin  eVs
Vs 
h
W
f  min
e
e
V s = (h/e) f – Wmin / e
Wmin = 2.3 eV
fc = 5.5  1014 Hz
work function Wmin (V)
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Symbol Physical Quantity
SI Unit
Sign
(yes /
no)
h
Planck's constant
J.s
no
f
Frequency of electromagnetic radiation
Hz
no
fc
Cut-off (critical ) frequency of electromagnetic
radiation or threshold frequency
Hz
no
m
Mass of electron ejected from metal
kg
no
v
vmax
Speed of electron ejected from metal
-1
m.s
no
Max speed of electron ejected from metal
m.s-1
no
W
Energy required to remove electron from metal
J
no
Wmin
Work function: min energy required to remove
electron from metal
J
no
N.s
no
p
Momentum of electron ejected from metal
K
Kinetic energy of electron ejected from metal
J
no
Max kinetic energy of electron ejected from metal
J
no
e
Electron charge
C
no
Vs
Stopping voltage (voltage required to prevent any
ejected electrons from reaching the collector)
V
no
Energy required to stop electrons from reaching
collector
J
no
Kmax
e Vs
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p2.06
p2.62
p2.50
p2.72
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p2.66
p2.08
p2.90
p2.95
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Particle Model
Experiments – blackbody radiation curves & photoelectric effect could not be explained
using wave model for light
Particle (photon)
Planck’s constant h = 6.63410-34 J.s
E=hf
ELECTRON Diffraction: Double Slit
-2
Irradiance IRR ( W.m )
1
0.8
0.6
0.4
0.2
0
-4
Do problems
-3
-2
-1
0
1
screen position x (mm)
2
3
4
p2.61
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