1. THE PHOTOELECTRIC EFFECT

2. Albert Einstein 1879-1955 Nobel Prize in Physics 1921
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Experimental set-up
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4. When red light is incident on a clean metal surface:
no electrons are released,
however long light is shone onto it,
however intense the light source is.

5. When UV light is incident on a clean metal surface:
electrons are released instantaneously,
however weak the light source.

6. Classically this cannot be explained because:
If red light is shone onto the metal surface
for long enough some electrons should
gain sufficient energy to enable them to escape.

7. Einstein put forward a theory:
Light energy is quantised.
Light consists of a stream of particles
called photons.
The energy of each photon (E) depends
on the frequency (f ) of the light.
h x f E=

8. h
is planck's constant
red light has a smaller
frequency
Frequency increasing
than violet light

9. Red light photons therefore
E = h x f
Photon energy
Red light photons therefore
have less energy
than violet light photons
and even less than UV photons

10. ONE
PHOTON
GIVES ALL ITS ENERGY e
TO ONE ELECTRON

11. e e e e e e e surface electrons Clean metal surface
A photon of red light gives an electron insufficient energy to
enable it to escape from the surface of the metal.
Red light photon
Clean metal surface e e e e e e e
surface electrons
No electrons are released from the metal surface

12. e e e e e e e surface electrons Clean metal surface
A photon of UV light gives an electron sufficient energy to
enable it to escape from the surface of the metal.
UV photon
Clean metal surface e e e e e e e
surface electrons
Electrons are released instantaneously. Each photon releases an electron This is called photoemission.

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Experimental set-up
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Intense
Triple Intensity
Double Intensity
Initial Intensity
The number of photoelectrons is proportional to the incident light intensity (frequency is constant)
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Freq
The applied voltage or stopping potential depends on the frequency:
– Higher frequencies generate higher energy electrons
• Classical theory: cannot explain this.
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Different metals
No electrons are produced below a certain frequency no matter
the intensity of the incident light.
• Classical theory: cannot explain this either !
Stopping potential
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Takes Planck’s ideas a step further
Suggests that the electromagnetic radiation is quantized.
Quanta of light have energy E=hf (photons)
– h: Planck’s constant
– Travels at the speed of light: c = fλ
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Photoelectric effect: when a photon collides with an electron, it gives away all its energy (some of which is transferred to the electron as kinetic energy)
hf = Φ + EK
Energy of the incoming photon
Kinetic Energy of the electron
Work function (energy necessary
for the electron to escape the material)
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( h = 6.62 x J.s )
( 1eV = 1.6 x J )
hf = Φ + EK
What is the threshold frequency for the photoelectric effect on lithium (Φ =2.93eV)? 1. f= 5.41 x 1017 Hz 2. f= 7.08 x 1014Hz 3. f= 2.01 x 106 Hz
What is the stopping potential if the wavelength of the incident light is 400nm?
1. V= 2.72 x V
2. V= V
3. V= 0.17 V
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