1. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 1 1. Photoelectric Effect and Phenomenon V A (1) Experimental Setup and Phenomenon

2. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 2 (2) Discoveries (2a) Current linearly proportional to the intensity.

3. 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

4. 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 (2c) Stopping Voltage O

5. 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

6. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 6 2. “Photon”, Einstein Equation (1) “light quanta” hypothesis Light comes in chunks (composed of particle-like “photon”), each light quanta has energy (2) Einstein photoelectric equation Escape work depends on material

7. 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 ( )

8. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 8 Stopping voltage V A Applied reverse stopping voltage stops electrons

9. 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)! thrshold frequency Threshold frequency: No lag: photon energy ( ) is absorbed by a electron and the electron then emits without time delay

10. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 10 (3) Measurement of Planck const. Stopping voltage vs. frequency O

11. 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.

12. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 12 Solution:

13. 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 Technology Photo-relay circuit, Automatic counter, measuring device etc.

14. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 14

15. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 15 4. Wave-particle Duality of Light Relativistic energy- momentum relation (2) particle:, photo-electric effect etc. (1) wave:diffraction and interference photon

16. 15-2 Photoelectric Effect Wave-particle Duality of Light Chap 15 Quantum Physics Physics 16 Particle character Wave character photon