1. Chapter 4 The Origin and Nature of Light

2. What do you think?
How hot is a “red hot” object compared to objects glowing with other colors?
What color is the Sun?

3. What can we learn by analyzing starlight?
A star’s temperature

4. increasing temperature
Peak color (wavelength) shifts to shorter wavelengths as an objects is heated
increasing temperature

5. Peak color (wavelength) emitted depends on an object’s temperature

6. Peak color (wavelength) shifts to shorter wavelengths as an objects is heated
hotter object
cooler object

7. The intensities of different emitted colors reveal a star’s temperature
Wien’s law
wavelength is inversely proportional to temperature
lmax = (2.9 x 10-3) / TKelvin

8. wavelength is inversely proportional to temperature
lmax = (2.9 x 10-3) / TKelvin

9. What color is our 5800K Sun?
The Sun emits all colors (wavelengths of electromagnetic radiation); however, the colors it emits most intensely are in the blue-green part of the spectrum.

10. What can we learn by analyzing starlight?
A star’s temperature
A star’s chemical composition

12. Each chemical element produces its own unique set of spectral lines when it burns

13. The Sun’s Spectrum

15. The brightness of spectral lines depend on conditions in the spectrum’s source
Law 1 A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors with without any specific spectral lines. (This is a black body spectrum.)

16. The brightness of spectral lines depend on conditions in the spectrum’s source
Law 2 A hot, rarefied gas produces an emission line spectrum - a series of bright spectral lines against a dark background.

17. The brightness of spectral lines depend on conditions in the spectrum’s source
Law 3 A cool gas in front of a continuous source of light produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.

18. Absorption Spectrum of Hydrogen Gas

19. Kirchhoff’s Laws

20. But, where does light actually come from?
Light comes from the movement of electrons in atoms

21. An atom consists of a small, dense nucleus surrounded by electrons

22. Atomic Vocabulary The nucleus contains protons and neutrons
All atoms with the same number of protons have the same name (called an element)
Atoms with varying numbers of neutrons are called isotopes.
Atoms with a varying numbers of electrons are called ions.

23. Spectral lines occur when an electron jumps from one energy level to another

25. What can we learn by analyzing starlight?
A star’s temperature
A star’s chemical composition
A star’s movement
Barnard’s Star

26. Spectral lines shift due to the relative motion between the source and the observer

27. Doppler Shifts
Red Shift: The distance between the observer and the source is increasing
Blue Shift: The distance between the observer and the source is decreasing

28. The Doppler shift allows astronomers to measure radial velocity

30. What can we learn by analyzing starlight?
A star’s temperature
by peak wavelength
A star’s chemical composition
by spectral analysis
A star’s radial velocity
from Doppler shifts

31. What did you think?
How hot is a “red hot” object compared to objects glowing with other colors?
Of all objects that glow from heat stored or generated inside them, those that glow red are the coolest.
What color is the Sun?
The Sun emits all colors (wavelengths of electromagnetic radiation). The colors it emits most intensely are in the blue-green part of the spectrum.

32. Self-Check
1: State the Stefan-Boltzmann law and Wien’s law and explain their meaning in the context of blackbody radiation and temperature determination.
2: Describe the evidence for the particle nature of light and indicate how the energy per photon is related to the wavelength and frequency in the wave model.
3: State Kirchhoff’s three laws of spectral analysis and indicate what information is derived about the nature of the light source in each case.
4: Describe the Bohr model of the atom in terms of its constituents and their distribution. Explain how spectral lines can be produced by a low-density gas.
5: Describe how spectroscopic analysis provides information about the chemical composition of celestial objects and indicate for which part of the object the information is valid.
6: Indicate how the numbers of protons, neutrons, and electrons are used to define elements, ions, and isotopes.
7: Describe the origin of line series in the hydrogen atom and explain why the Balmer lines occur at visual wavelengths but the other line series do not.
8: Define excitation and ionization in the context of the Bohr model of atoms.
9: Describe how the Doppler shift reveals the radial motion of the stars.