1. 5.4 Learning from Light Our goals for learning
What are the three basic types of spectra?
How is energy stored in an atom?
How does light tell us what things are made of?

2. How do astronomers study light?
Astronomers use a spectrograph – an instrument that separates light by wavelength and records the spectrum
Uses a prism or diffraction grating

3. What are the three basic types of spectra?
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of these three basic types

4. Continuous Spectrum
An unbroken band of colors produced by glowing solids, liquids & high pressure gases.
Ex: The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption

5. Emission Line Spectrum
Produced by hot gases under low pressure.
EX: A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines

6. Absorption Line Spectrum
Produced when a continuous spectrum of light is passed through a cooler, low pressure gas.
EX: A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum

7. Stellar Spectra

8. How does light tell us what things are made of?
Spectrum of the Sun

9. Sun’s Spectrum
hydrogen
sodium
magnesium
what is in the Sun?
iron

10. Remember, energy stored in atoms
Electrons are restricted to particular energy levels
The lowest level is called the ground state
Higher levels are called excited states

11. The energy is measured in electron-Volts. 1 eV = 1.6 x 10 – 19 J
Excited States
Ground State
The energy is measured in electron-Volts.
1 eV = 1.6 x 10 – 19 J

12. The only allowed changes in energy are those corresponding to a transition between energy levels
Not Allowed
Allowed

13. Excited States Ground State
Electrons gain energy (by absorbing light) and jump to a higher level
Electrons lose energy ( by emitting light) and drop to a lower level

15. Chemical Fingerprints
Each type of atom has a unique set of energy levels
Each transition corresponds to a unique photon energy, frequency, and wavelength
Energy levels of Hydrogen

16. Chemical Fingerprints
Downward transitions produce a unique pattern of emission lines

17. Chemical Fingerprints
Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths

18. Chemical Fingerprints
Each type of atom has a unique spectral fingerprint
Observing the fingerprints in a spectrum tells us which kinds of atoms are present

19. Energy Levels of Molecules
Molecules have additional energy levels because they can vibrate and rotate

20. Energy Levels of Molecules
Spectrum of Molecular Hydrogen
The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated
Many of these molecular transitions are in the infrared part of the spectrum

21. Video – How do we know what the air on other planets is made of?

22. What have we learned? What are the three basic type of spectra?
Continuous spectrum, emission line spectrum, absorption line spectrum
How does light tell us what things are made of?
Each atom has a unique fingerprint.
We can determine which atoms something is made of by looking for their fingerprints in the spectrum.