1. Light and Spectroscopy
AST 111 Lecture 11

2. Light Charges interact via electric and magnetic forces
Light is a repetitive disturbance in these forces!
Electromagnetic wave
A form of energy
Depending on conditions, light can also act like a particle
A photon

3. The Electromagnetic Spectrum
Higher Energy
Lower Energy
Blue light: 400 nm Red light: 700 nm

4. Light Wavelength relates to COLOR Shorter wavelengths have more energy
Higher-energy light interacts more strongly with matter
Infrared can pass through dust
X-Rays can pull electrons off of atoms

5. Light and Atoms
Light comes from electrons moving to lower energy levels in an atom
Atoms can also absorb light, promoting an electron to a higher level

6. Emission and Absorption
Because the energy levels in atoms are specifically spaced apart:
A given type of atom can only emit and absorb specific colors of light

7. Energy Level Diagrams

8. Light and Atoms Different atoms have different energy levels
A given type of atom emits and absorbs specific colors of light

9. Light and Atoms
Atomic spectra therefore provide a wealth of information about the physical properties, especially chemical composition, of an object.

10. Spectra
A prism spreads light into its different colors

11. Spectra

12. Spectra We will consider three types of spectra: Emission Absorption
Continuous

13. Spectra Emission and Absorption spectra:
An atom can both emit and absorb light
We consider a gas that is not dense
If we don’t, the atoms interact and alter the energy levels
This ruins the unique spectral fingerprint of the individual atoms

14. Emission Spectra
If a gas has enough microscopic energy (high enough temperature):
Collisions between atoms will transfer energy to electrons
Electrons then drop to a lower energy level, emitting a photon

15. Emission Spectra

16. An aside…
What does conservation of energy say will happen to the temperature of the gas?

17. Emission Spectrum

18. Absorption Spectra White light is shining through a cold gas cloud.
White light contains all colors of the spectrum.
The atoms in the gas cloud absorb photons with energies corresponding to differences in atomic energy levels
These colors are therefore removed from the spectrum

19. Absorption Spectra

20. Absorption Spectra

21. Continuous Spectra
Hot, dense objects emit essentially continuous spectra

22. Continuous Spectra
The spectrum of a hot dense object has a bump-shaped graph
The graph shows the brightness of each color (wavelength)
What determines the exact shape?

23. Continuous Spectra
The amount of energy emitted by such an object is given by the Stefan-Boltzmann law:
Brightness = sT4
The wavelength at which the peak occurs is given by Wien’s Law
Higher temperature object peaks at shorter wavelengths

24. Temperature Estimation
So we can use a glowing object’s color to estimate its temperature
Does this apply to the lava? How about an orange shirt?
Why or why not?

25. Spectrum of Mars
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26. Spectrum of Mars Is there evidence of continuous spectra?
Are there emission lines?
Absorption lines?

27. Spectrum of Mars The boxed region shows reflected sunlight
What color is Mars?
The Sun is a source of a continuous spectrum

28. Spectrum of Mars What causes the lower-energy bump? Does Mars glow?
Do we see its glow?

29. Spectrum of Mars
This is the signature of carbon dioxide.

30. Spectrum of Mars Emission lines
These come from hot gas in the atmosphere

31. Doppler Effect
Because of the wave nature of light, its frequency or wavelength change if the source moves toward or away from an observer.

32. Doppler Effect An object’s spectrum blueshifts if it moves toward us
An object’s spectrum redshifts if it moves away from us

33. Doppler Effect
Based on an object’s redshift or blueshift, we can ascertain how fast it is moving toward or away from us
We know where the lines should be when the object is at rest, so we can easily measure the shift