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15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 3 (2b) threshold frequency Threshold frequency depends on type of metal, but not on intensity For a given metal, electrons only emitted if frequency of incident light exceeds a threshold 0. 0 is called threshold frequency
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15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
1
1. Photoelectric Effect and Phenomenon
VA
(1) Experimental Setup and Phenomenon
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
2
(2) Discoveries
(2a) Current linearly proportional to the intensity.
1I2I
i
m1im2i
o0U U
12 II
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
3
(2b) threshold frequency 0
Threshold frequency depends on type of metal, but not on intensity
For a given metal, electrons only emitted if frequency of incident light exceeds a threshold0. 0 is called threshold frequency
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
4
Stopping voltage linearly related incident light frequency
(2d) Current appears with no delay
Applied reverse voltage that makes zero current is so-called stopping voltage , different metal has different
0U
0U
(2c) Stopping Voltage 0U
0U
0
sC nZ tP
O
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
5
For very low intensities, expect a time lag between light exposure and emission, while electrons absorb enough energy to escape from material
(3) Failures of Classical Theory
Threshold frequency
No time delay
Electrons should be emitted whatever the frequency ν of the light, so long as electric field E is sufficiently large
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
6
2. “Photon”, Einstein Equation(1) “light quanta” hypothesis
hε
Light comes in chunks (composed of particle-like “photon”), each light quanta has energy
(2) Einstein photoelectric equation
Wmh 2
21 v Escape work
depends on material
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
7
Theoretical Explanation:
Approximate escape work value of different metals (in eV)
Na Al Zn Cu Ag Pt
2.46 4.08 4.31 4.70 4.73 6.35
the greater the intensity, the more photons, the more photo-electrons, and hence the larger current ( )0
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
8
Stopping voltage
VA
Applied reverse stopping voltage stops electrons
0U
20 2
1 vmeU
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
9
Einstein’s theory successfully explained the photoelectric effect and won 1921 Nobel prize of physics (not for relativity)!
0hW hW0 thrshold frequency
Threshold frequency: 0
No lag: photon energy ( ) is absorbed by a electron and the electron then emits without time delay
h 0
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
10
WeUh 0
eW
ehU 0
(3) Measurement of Planck const.
Wmh 2
21 v
ehU 0 eUh
0
0U
0
Stopping voltage vs. frequency
O
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
11
E.g.-1. Consider a thin circular plane with radius , 1.0 m far from an 1W power light source. The light source emits monochromatic light with wave length 589 nm. Suppose the energy goes off all directions equally. Calculate the number of photons on the plate per unit time.
m100.1 3
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
12
111 s 104.7 hcE
hEN
172 sJ 105.2
π4
rSPE
Solution:
2623 m10π)m100.1(π S
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
13
Photomultiplier
Amplifier
Controller
light
Demo. of photo-relay
3. Applications in Modern TechnologyPhoto-relay circuit, Automatic counter, measuring device etc.
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
14
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
15
4. Wave-particle Duality of Light
20
222 EcpE Relativistic energy-momentum relation
hE (2) particle: , photo-electric effect etc.
(1) wave:diffraction and interference
pcEE ,00 photon
15-2 Photoelectric Effect Wave-particle Duality of Light
Chap 15 Quantum Physics
Physics
16
hE
hp
Particle character
Wave character
hch
cEp
pcEE ,00 photon