Light is a Wave, so What’s this about
Photons? The Need for Quantum Mechanics
Exam 2 Class Average = 14
4
3
2
1
5
0
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Score/20
Number of Students
Number of Students
5
4
Photoelectric Effect
Emission of electrons by an
illuminated metal
Blackbody Radiation
The electromagnetic radiation
emitted by a heated object
v/ c
Relativistic Relativistic
mechanics, quantum
El.-Mag.
mechanics
(1905)
(1927-)
Quantum
Classical
mechanics
physics
(1920’s-)
3
2
1
0
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
Improvement in score from Exam 1
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
lig
ht
e-
I
+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 (I)
“Classical” Method
Increase energy by
increasing amplitude
electrons
emitted ?
No
No
No
No
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 λ large ν)