1. Joel E. Tohline, Alumni Professor
ASTR Spring 2008
Joel E. Tohline, Alumni Professor
247 Nicholson Hall
[Slides from Lecture21]

2. Kirchhoff’s Laws
Hot dense gas produces a continuous spectrum (a complete rainbow of colors)
Hot transparent gas produces an emission line spectrum
Cool transparent gas in front of a source of continuous spectrum produces an absorption line spectrum.

4. Emission-Line “Fingerprints” of 7 Different Atoms or Molecules
These two shown in class

6. More About: Continuous Spectra from Hot Dense Gases (or Solids)
Kirchhoff’s 1st Law: Hot dense gas produces a continuous spectrum (a complete rainbow of colors)
A plot of light intensity versus wavelength always has the same general appearance (black body function):
Very little light at very short wavelengths
Very little light at very long wavelengths
Intensity of light peaks at some intermediate wavelength
But the color that marks the brightest intensity varies with gas temperature:
Hot objects are “blue”
Cold objects are “red”

7. More About: Continuous Spectra from Hot Dense Gases (or Solids)
Kirchhoff’s 1st Law: Hot dense gas produces a continuous spectrum (a complete rainbow of colors)
A plot of light intensity versus wavelength always has the same general appearance (black body function):
Very little light at very short wavelengths
Very little light at very long wavelengths
Intensity of light peaks at some intermediate wavelength
But the color that marks the brightest intensity varies with gas temperature:
Hot objects are “blue”
Cold objects are “red”
First, a reminder about various temperature scales …

8. Various Temperature Scales
°C = °K – 273°
°F = 1.8(°C) + 32°

9. Various Temperature Scales

10. More About: Continuous Spectra from Hot Dense Gases (or Solids)
Kirchhoff’s 1st Law: Hot dense gas produces a continuous spectrum (a complete rainbow of colors)
A plot of light intensity versus wavelength always has the same general appearance (blackbody function):
Very little light at very short wavelengths
Very little light at very long wavelengths
Intensity of light peaks at some intermediate wavelength
But the color that marks the brightest intensity varies with gas temperature:
Hot objects are “blue”
Cold objects are “red”

11. The Sun’s Continuous Spectrum
(Textbook Figure 5-12)

12. More About: Continuous Spectra from Hot Dense Gases (or Solids)
Kirchhoff’s 1st Law: Hot dense gas produces a continuous spectrum (a complete rainbow of colors)
A plot of light intensity versus wavelength always has the same general appearance (blackbody function):
Very little light at very short wavelengths
Very little light at very long wavelengths
Intensity of light peaks at some intermediate wavelength
But the color that marks the brightest intensity varies with gas temperature:
Hot objects are “bluer”
Cold objects are “redder”

16. Wien’s Law for Blackbody Spectra
As the textbook points out (§5-4), there is a mathematical equation that shows precisely how the wavelength (color) of maximum intensity varies with gas temperature.

17. More About: Continuous Spectra from Hot Dense Gases (or Solids)
The total amount of energy that is radiated by a dense hot gas varies with gas temperature:
Hot objects are a great deal brighter overall than cold objects

19. Stefan-Boltzmann Law for Blackbody Spectra
As the textbook points out (§5-4), there is a mathematical equation that shows precisely how the amount of energy emitted by a blackbody varies with gas temperature.

20. Doppler Effect (§5-9)
The measured frequency (wavelength) of light changes if the light source is moving toward or away from you
Light is “blue-shifted” is the object is moving toward you
Light is “red-shifted” if the object is moving away from you
Direct analogy with the behavior of sound waves