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

Modern Physics

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.

Blackbody and temperature Peak gives main color

Black Body Radiation Intensity of blackbody radiation Planck’s expression h =  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):

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

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

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

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

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 ! )

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

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

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

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

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