Physics 1202: Lecture 30Today’s Agenda

• Announcements:

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

–Team in class Team in class

• HW 9 next FridayHW 9 next 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 radiation• Heating an object

– Motion of closely spaced atoms/molecules produce E&M waves in a continuous range of or

• Blackbody radiation– The hotter an object the more white it appears

» E.g. the sun

– Not all wavelength are emitted equally though

» Very hot appears more blue (e.g. Sirius)

» Colder means less blue: object appears more red (e.g. fire)

» Earth at 300K: mainly infrared waves.

Blackbody radiation• In late 19th century

– Study of relationship ot temperature T and

– Wilhelm Wien (1893):

maxT = 2.898 X 10-3 mK

max

(emissivity) = 1 for perfect blackbody (absorbs all: appears black) = 0 for perfect reflective surface

• Stefan-Boltzmann law:– Power radiated by a surface of

area A and temperature T

P = A T4

5.67 X 10-8 W/(m2K4)

Blackbody and temperature• Peak gives main

color

Star temperature• From star color

– Can determine temperature assuming it is a blackbody

Black Body Radiation

Intensity of blackbody radiationClassical Rayleigh-Jeans law forradiation emission (1905)

Ultraviolet catastrophe

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

Atomic Spectra

a) Emission line spectra for hydrogen, mercury, and neon;b) Absorption spectrum for hydrogen.