15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 1 1....

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



Physics

2

(2) Discoveries

(2a) Current linearly proportional to the intensity.

1I2I

i

m1im2i

o0U U

12 II



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



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



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



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



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



Physics

8

Stopping voltage

VA

Applied reverse stopping voltage stops electrons

0U

20 2

1 vmeU



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



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



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



Physics

12

111 s 104.7 hcE

hEN

172 sJ 105.2

π4

rSPE

Solution:

2623 m10π)m100.1(π S



Physics

13

Photomultiplier

Amplifier

Controller

light

Demo. of photo-relay

3. Applications in Modern TechnologyPhoto-relay circuit, Automatic counter, measuring device etc.



Physics

14



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



Physics

16

hE

hp

Particle character

Wave character

hch

cEp

pcEE ,00 photon

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15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 1 1....

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