Physics 1202: Lecture 31 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like...

Physics 1202: Lecture 31Today’s Agenda

• Announcements:

• Extra creditsExtra credits–Final-like problemsFinal-like problems

–Team in class 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 = 6.626 10-34 J · s : Planck’s constant

Assumptions: 1. Molecules can have only discrete values of energy En;

2. The molecules emit or absorb energy by discrete packets - photons

Max Planck (1899):

Quantum energy levels

Energy

E

0

1

3

4

5

2

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: m0 = 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 itsde 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