ElectroMagnetic
Spectrum
Quick Quiz:
Why do we use infrared
waves and not X-rays to
heat our homes?
A. X-ray wavelengths
are too short
B. X-ray wavelengths are
too long
C. We use microwaves to
heat our homes
D. Physical law says that Xrays can only be used
for medicine
Discovery: Heinrich Hertz
Photoelectric Effect



When light is incident on certain metallic
surfaces, electrons are emitted from the
surface
The effect was first discovered by Hertz
The successful explanation of the effect was
given by Einstein in 1905
For what did Einstein receive the Nobel Prize in 1921?
a. Theory of Special Relativity
b. Theory of General Relativity
c. Theory of Quantum Gravity
d. Theory of the photoelectric effect
e. Einstein never received the Nobel Prize
Hertz’s Spark Gap Generator
Back in 1887…

Hertz clarified Maxwell’s electromagnetic theory of
light:
 Proved that electricity can be transmitted in
electromagnetic waves.
 Established that light was a form of electromagnetic
radiation.
 First person to broadcast and receive these waves.
Electrons sometimes are Emitted from a
Metal when Light Shines on the Metal
f
Classical? Or Quantum?
“Classical” Method
Increase energy by
increasing amplitude
electrons
emitted ?
No
No
No
No
What if we try this ?
Vary wavelength, fixed amplitude
electrons
emitted ?
No
Yes, with
low KE
Yes, with
high KE
No electrons were emitted until the frequency of the light
exceeded a critical frequency, at which point electrons were
emitted from the surface!
(Recall: small   large f)
1
Photoelectric Effect
Schematic
I
e+V _



When light strikes E,
photoelectrons are
emitted
Electrons collected at
C and passing
through the
ammeter are a
current in the circuit
C is maintained at a
positive potential by
the power supply
Photoelectric Effect (II)
 Electrons are attracted to the (positively charged) nucleus by the
electrical force
 In metals, the outermost electrons are not tightly bound, and can
be easily “liberated” from the shackles of its atom.
 It just takes sufficient energy…
Classically, we increase the energy
of an EM wave by increasing the
intensity (e.g. brightness)
Meter on Box
Energy  A2
Digital Meter
But this doesn’t work ??
Quantum of Light Energy
Enter Mr. Genius!
 Light is acting like a particle
 The light particle must have sufficient
energy to “free” the electron from the atom.
 Increasing the Amplitude is simply increasing the
number of light particles, but its NOT increasing the
energy of each one!
 However, if the energy of these “light particle” is related to
their frequency, this would explain why higher frequency light
can knock the electrons out of their atoms, but low frequency
light cannot…
 In this “quantum-mechanical” picture, the energy of the
light particle (photon) must overcome the binding energy of
the electron to the nucleus.
 If the energy of the photon exceeds the binding energy, the
electron is emitted with a KE = Ephoton – Ebinding.
 The energy of the photon is given by E=hfwhere the
constant h = 6.6x10-34 [J s] is Planck’s constant.
“Light particle”
After Collision
Before Collision
Photons
 Quantum theory describes light as
a particle called a photon
If Photoelectric Effect were Classical
The following graphs illustrate what the
wave theory predicts will happen:
 According to quantum theory, a
photon has an energy given by
E = hf = hc/
h = 6.6x10-34 [J s] Planck’s constant,
after the scientist Max Planck.
 The energy of the light is proportional to the frequency (inversely
proportional to the wavelength) ! The higher the frequency (lower
wavelength) the higher the energy of the photon.
 10 photons have an energy equal to ten times a single photon.
 Quantum theory describes experiments to astonishing precision,
whereas the classical wave description cannot
Icurrent
Icurrent
Ilight intensity
Voltage
Icurrent
frequency of light
2
Photoelectric Effect is Quantum Effect
We now show in green what actually
happens:
V-stop
Ilight intensity
Voltage
Icurrent
f-co
In addition, we see a connection between
the stopping voltage and frequency
above f-co (frequency of cut-off):
Quick Quiz
If V = hf, the slope of the graph is:
V-stop
Icurrent
Icurrent
Stopping Voltage depends on f
f-co
frequency
A.
B.
C.
D.
E.
1/h
V
h
f
1/f
frequency of light
3