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The University of Sydney
School of Physics
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The Photoelectric Effect
Albert Einstein won his Nobel Prize in Physics for his insight in to the photoelectric effect. His
understanding of this phenomenon was one of the milestones in the development of quantum
theory and introduced the world to the concept of wave-particle duality.
Photon
electron
A
Current flows in circuit
when light shines
Waves? Particles? Waves and particles?
Light definitely does behave like a wave: it diffracts, refracts and
interferes, which are all wave properties. Light also seems to
behave like a particle, like a lump: it can knock electrons off
surfaces and transfer energy to other objects in packets of E = hf,
just like a particle would.
Observing the Photoelectric Effect
In this experiment you can observe the photoelectric effect as
certain frequencies of light knock electrons out of a metal surface. When the electroscope is
charged, shine dim, ‘white’ light source (the desk lamp on ‘low’) onto the metal plate.
Switch the lamp to its brightest setting. Do you observe any change in the charge on the
electroscope?
Predict
Observe
Explain
The University of Sydney
School of Physics
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Now shine the UV lamp on the metal plate. What do you observe?
Predict
Observe
Explain
Measuring the photoelectric ‘stopping voltage’
In this experiment you again observe the photoelectric effect, but this time you’re going to
measure the amount of energy the ejected electrons receive from the photons. energy E = hf.
This means that electrons knocked out from the surface of a material will all have roughly the
same amount of kinetic (moving) energy when they leave:
Filter colour
Yellow
Green
Blue
Violet
Ultraviolet
Filter
frequency (Hz)
5.19 x 1014
5.49 x 1014
6.88 x 1014
7.41 x 1014
8.20 x 1014
Stopping
voltage (V)
You now have the necessary information to determine the work function of the phototube’s
cathode, and to find a value for Planck’s constant.
How do you do this?
•
The electrons had energy K = hf – W when they left the surface.
•
Electrons passing through the voltage V applied to the phototube will lose an amount of
energy equal to qv
•
At the stopping voltage, the electrons just don’t make it to the anode, they have lost all
their initial energy — so for this voltage, qV = K.
•
This means qV = hf – W, which we rewrite as V = (h/q) f – (W/q)
The University of Sydney
School of Physics
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This is a linear equation, similar to y = mx + c. So you can make a graph of V vs. f. It should
be a straight line, with a slope equal to h/q and a y-intercept equal to W/q.
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