1. The Light Fantastic! Astronomy relies on messages from all kinds of light

2. Light Light from celestial objects carries data Must transform into information by models Measure: Direction, total light (flux), colors (spectra)

3. Light Wave model: Wave properties (wavelength, frequency, speed) Full range of wavelengths: electromagnetic (E&M) spectrum All (radio to gamma rays) travel at same speed (about 300,000 km/s)

4. E&M Spectrum Radio (longest, up to km), Infrared, Visible, Ultraviolet, X- rays, Gamma rays (shortest, size of atom’s nucleus) Different detectors for each type

5. Radiative Energy Light carries radiative energy –Higher frequencies, greater energies –Shorter wavelengths, greater energies Energy directly proportional to frequency, inversely proportional to wavelength

6. Light Particle model: Light comes in discrete chunks (photons) Atoms emit/absorb photons: produce spectrum unique to each element Analyze spectra to find key physical properties

7. Spectra Continuous: Unbroken range of colors (wavelengths) Bright line: Individual bright lines at certain colors Dark line: Individual dark lines at certain colors

8. Kirchhoff’s Rules 1. Hot and opaque solid, liquid, or gas emits a continuous spectrum

9. Kirchhoff’s Rules 2. Hot, transparent gas emits bright lines. Colors of the lines depend on the chemical composition of the gas.

10. Kirchhoff’s Rules 3. A continuous spectrum passing through a cooler, transparent gas will display dark lines. The colors of the line relate to the chemical composition of the gas.

11. Atoms and Light Electrons orbit nuclei at special discrete energy levels Electron jumps between levels create (emission) or destroy (absorption) photons Visible as bright/dark lines

12. Atoms and Energy Levels Each atom has unique set of energy levels Electron moves up => absorb energy; moves down, emits energy; one photon per jump Photon’s energy equal to difference between energy levels