HW 5 (Ch. 6) Quiz Question 1
HW 5 (Ch. 6) Quiz Question 2
A metal surface ejects electrons when
illuminated with green light but not with
yellow light. Which of the following colors
will not eject electrons from the metal?
A. Blue
B. Violet
C. Red
Before digital photography people worked in
“dark” rooms to print photos. The rooms
usually had a red light so they were not totally
dark. Why red and not green or blue
illumination?
A.
Red light has lower energy photons than
can effect most photographic paper.
B.
Red light is easier on he eyes
C.
Green and blue light bulbs are more
expensive than red.
D.
The eye is more sensitive to red light so the
room may be illuminated at a lower
intensity.
Discovery: Heinrich Hertz and Phillip
Lenard
HW 5 (Ch. 6) Quiz Question 3
You all have experience with a good mirror.
Think about what you see in a mirror. Now
consider that about 80% of the light
incident on the mirror is reflected. How do
you know 20% of the incident photons are
absorbed, and not that 20% of the energy
of each photon is lost?
A. We don’t – it would take more
measurements.
B. The color of the light does not change
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.
Lenard Goes Further…
His assistant, Phillip Lenard, explored the
effect further. He built his own apparatus
called a “phototube” to determine the
nature of the effect:
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Light is a Wave, so What’s this about
Photons? The Need for Quantum Mechanics
Lenard’s Photoelectric Apparatus:

Photoelectric Effect
 Emission of electrons by an
illuminated metal
v/c
Relativistic
mechanics,
El.-Mag.
(1905)
Relativistic
quantum
mechanics
(1927-)
Classical
physics
Quantum
mechanics
(1920’s-)
h /s
Electrons sometimes are Emitted from a
Metal when Light Shines on the Metal
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

Received Nobel Prize in 1921 for paper on
electromagnetic radiation, of which the photoelectric
effect was a part
Photoelectric Effect
Schematic
Photoelectric Effect-What’s Going On?
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
“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 )
2
Photoelectric Effect-Classical Physics
Enter a real Einstein
 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.
 An alternate view is that light is acting like a particle—one light particle is
needed to eject one electron
 It just takes sufficient energy…
 The light particle must have sufficient energy to “free” the
electron from the atom.
Classically, we increase the energy
of an EM wave by increasing the
intensity (e.g. brightness)
Energy  A2
But this doesn’t work ??
 Increasing the Amplitude is simply increasing the number
of light particles, and two light particles can’t add to one electron’s energy
at the same time!
 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…
Quantum Mechanics
 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=hwhere the
constant h = 6.6x10-34 [J s] is Planck’s constant.
“Light particle”
Before Collision
After Collision
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