2. Light as a wave in an abstract electromagnetic field, or as a stream of massless particles called photons

3. Units of Wavelength UnitSymbolLength centimetercm10 -2 meters Angstrom 10 -8 centimeters nanometernm10 -9 meters micrometernm10 -6 meters

4. c = 3 x 10 10 cm/second h = 4.135 x 10 -15 eV-sec

5. Spectrum of Electromagnetic Radiation Region Wavelength (Angstroms ) Wavelength (centimeters) Frequency (Hz) Energy (eV) Radio > 10 9 > 10< 3 x 10 9 < 10 -5 Microwave 10 9 - 10 6 10 - 0.01 3 x 10 9 - 3 x 10 12 10 -5 -0.01 Infrared 10 6 - 70000.01 - 7 x 10 -5 3 x 10 12 - 4.3 x 10 14 0.01 - 2 Visible7000 - 40007 x 10 -5 -4 x 10 -5 4.3 x 10 14 -7.5 x 10 14 2 - 3 Ultraviolet 4000 - 104 x 10 -5 - 10 -7 7.5 x 10 14 - 3 x 10 17 3 - 10 3 X-Rays 10 - 0.110 -7 - 10 -9 3 x 10 17 - 3 x 10 19 10 3 - 10 5 Gamma Rays < 0.1< 10 -9 > 3 x 10 19 > 10 5

14. Wavelength Characteristic Object Gamma-Ray Compact objects in collision (neutron stars,...)? X-RaysNeutron stars Ultraviolet Hot stars, quasars VisibleStars Infrared Red giant stars, galactic nuclei Far-IR Protostars, dust, planets Millimeter Cold dust, molecular clouds cm Radio HI 21-cm line, pulsars