1. Light and Spectroscopy

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. 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

4. The Electromagnetic Spectrum Blue light: 400 nm Red light: 700 nm Higher EnergyLower Energy

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.  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

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 =  T 4  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. Doppler Effect  Because of the wave nature of light, its frequency or wavelength change if the source moves toward or away from an observer.

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

27. 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