1. Physics 1202: Lecture 31 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like problems –Team in class HW 9 this FridayHW 9 this Friday Modern physics

2. Modern Physics

3. Quantization Physical quantities come in small but finite quantities –Quantum (or quanta for many of them) –Not continuous Atomic Spectra: a)Emission line spectra for hydrogen, mercury, and neon; b)Absorption spectrum for hydrogen.

4. Blackbody and temperature Peak gives main color

5. Black Body Radiation Intensity of blackbody radiation Planck’s expression h = 6.626  10 -34 J · s : Planck’s constant Assumptions: 1. Molecules can have only discrete values of energy E n; 2. The molecules emit or absorb energy by discrete packets - photons Max Planck (1899):

6. Quantum energy levels Energy E 0 1 3 4 5 2 n hf 2hf 3hf 4hf 0 5hf

7. Photoelectric effect In 1887, Heinrich Hertz –shining ultra-violet light on metal in vacuum –If V not large enough, no current

8. Photoelectric effect Kinetic energy of liberated electrons is where  is the work function of the metal

9. Photoelectric effect Explained by Einstein in 1905 –Based on quantum of light (Planck) –Nobel Prize in 1914

10. Photon properties Recall (for electromagnetic wave) E = pc Quantization (Planck): E = hf = hc / So  = h / p Recall from relativity Conclusion: m 0 = 0 (photons have no mass ! )

11. Compton effect In 1920’s, Arthur Compton experiments with X-rays –Wavelength longer after scattering –Using quantization he derived C : Compton wavelength

12. The waves properties of particles In 1924, Louis de Broglie postulate: because photons have both wave and particle characteristics, perhaps all forms of matter have both properties Momentum of the photon De Broglie wavelength of a particle

13. Example: An accelerated charged particle An electron accelerates through the potential difference 50 V. Calculate its de Broglie wavelength. Solution: Energy conservation Momentum of electron Wavelength

14. Birth of quantum mechanics Erwin Schrödinger –Wave function & Hamiltonian Werner Heisenberg –Uncertainty principle

15. 5 steps methods Draw and list quantitites Concepts and equations needed Solve in term of symbols Solve with numbers Checks values and units