1. Physics 1202: Lecture 30 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like problems –Team in class HW 9 next FridayHW 9 next 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 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.

5. Blackbody radiation In late 19 th century –Study of relationship ot temperature T and –Wilhelm Wien (1893): max T = 2.898 X 10 -3 m  K 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 T 4  5.67 X 10 -8 W/(m 2 K 4 )

6. Blackbody and temperature Peak gives main color

7. Star temperature From star color –Can determine temperature assuming it is a blackbody

8. Black Body Radiation Intensity of blackbody radiation Classical Rayleigh-Jeans law for radiation emission (1905) Ultraviolet catastrophe

9. 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):

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

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

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

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

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