BLACK BODY RADIATION AND THE EXPERIMENTAL BASIS FOR

Josef Stefan (1835-1893) • Total energy radiated proportional to T4 Question: Suppose temperature raised from 300K (room temp) to 6000K (sun) By what factor does the energy radiated increase? A. B. C. D.

16 16,000 90,000 160,000

Josef Stefan (1835-1893) • Total energy radiated proportional to T4 Question: Suppose temperature raised from 300K (room temp) to 6000K (sun) By what factor does the energy radiated increase? A. B. C. D.

16 16,000 90,000 160,000 Answer: (6000/300)4 = 204=160,000

Question: Wavelength of emitted radiation (light) changes as objects get hotter. • 1. Hotter -> Longer Wavelength • 2. Hotter -> Shorter Wavelength

Question: Wavelength of emitted radiation (light) changes as objects get hotter. • 1. Hotter -> Longer Wavelength • 2. Hotter -> Shorter Wavelength

Steam Radiator 400K Very Hot Stove 900K Lava ~2000K Sun 6000K

IR Dull Red (transition to visable) Glowing (most radiation still in IR) Most radiation visable

Wilhelm Wien

Wilhelm Wien (18643-1928) Nobel Prize 1911

Example of Black Body Spectra for different temperatures

Below is a photo of three stars. The light emitted by these stars is thermal radiation. Which of these stars is the hottest? The coolest? A. Red, yellow B. Red, blue C. Yellow, blue D.Blue, red E. Blue, yellow © 2007 W.W. Norton & Company, Inc. Physics for Engineers and Scientists

10

Below is a photo of three stars. The light emitted by these stars is thermal radiation. Which of these stars is the hottest? The coolest? A. Red, yellow B. Red, blue C. Yellow, blue D.Blue, red E. Blue, yellow © 2007 W.W. Norton & Company, Inc. Physics for Engineers and Scientists

11

Spectrum of high energy particle radiation from space does not follow a black body spectrum Cosmic Ray Flux

Above 1020 eV Very low flux ~ 1 particle per km2/sr/century

GeV 109

TeV 1012

PeV 1015

EeV 1018

1 Joule

ZeV 1021

What is the best known example of a black body source?

What is the best known example of a black body source? Hint

Temperature = 2.7 K

Cosmic Microwave Background (Radiation from Big Bang! T=2.725K. The theoretical curve obscures the data points and the error bars.

Astrophysical Journal, 473, 576

Black Body Radiation and the experimental basis for Quantum Theory

What is the energy of a “quanta” of RED light? 660 nm wavelength in units of electron volts?

E=hν h=6.62x10-34 Js

What is the energy of a “quanta” of RED light? 660 nm wavelength in units of electron volts?

E=hν h=6.62x10-34 Js ν=c/λ=3x108m/s/660x10-9m =4.54x1014 s-1 E= 6.62x10-34Js x 4.54x1014 s-1 E= 3.01 x 10-19 J 1eV= 1.602 x10-19 J E=3.01x10-19J/1.602x10-19J/eV E=1.88 eV

What is the energy of a “quanta” of RED light? 660 nm wavelength in units of electron volts?

E=hν h=6.62x10-34 Js ν=c/λ=3x108m/s/660x10-9m =4.54x1014 s-1 E= 6.62x10-34Js x 4.54x1014 s-1 E= 3.01 x 10-19 J 1eV= 1.602 x10-19 J E=3.01x10-19J/1.602x10-19J/eV E=1.88 eV

What is the energy of a “quanta” of RED light? 660 nm wavelength in units of electron Volts

Easier Way to Solve this E=hc/λ hc=1240 eV nm (useful constant to remember) E=(1240/660)eV E= 1.88 eV

Question: What is the energy quantization of a grandfather clock? Hint:

Question: What is the energy quantization of a grandfather clock? Hint:

Question: What is the energy quantization of a grandfather clock? Hint:

E=nhν for n=1, ν=1Hz=1s-1 E= 6.6x10-34J

E=nhν for n=1, ν=1Hz=1s-1 E= 6.6x10-34J

How is the quantization realized?

E=nhν for n=1, ν=1Hz=1s-1 E= 6.6x10-34J

How does this quantization translate into quantization of the pendulum displacement (height)?

E=nhν for n=1, ν=1Hz=1s-1 E= 6.6x10-34J

How does this quantization translate into quantization of the pendulum displacement (height)? E=mgH=6.6x10-34J

H

H=6.6x10-34J/(1kg 10m/s2)=6.6x10-35m Too small to measure (size of an atom is about 10-8 m)