Black Body Radiation and

the experimental basis for Quantum Theory

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. 16 B. 16,000 C. 90,000 D. 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. 16 B. 16,000 C. 90,000 D. 160,000 Answer: (6000/300)4 = 204=160,000

Steam Radiator 400K IR Very Hot Stove 900K Dull Red (transition to visable) Lava ~2000K Glowing (most radiation still in IR) Sun 6000K Most radiation visable

Wilhelm Wien

Wilhelm Wien (18643-1928) Nobel Prize 1911

Example of Black Body Spectra for different temperatures

© 2007 W.W. Norton & Company, Inc. Physics for Engineers and Scientists 9

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

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

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

1 Joule

Cosmic Ray Flux

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

Astrophysical Journal, 473, 576

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

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

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:

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

E=mgH=6.6x10-34J H=6.6x10-34J/(1kg 10m/s2)=6.6x10-35m H

Too small to measure (size of an atom is about 10-8 m)