1. Importance of electromagnetic radiation

• Almost everything we know about the universe comes from observations of radiation (remote sensing).
• Light is just one of several kinds (wavelength bands) of electromagnetic radiation
• Most kinds of e-m radiation cannot penetrate the Earth's atmosphere

• The revolution in astronomy is largely due to our ability to observe the sky at different wavelength bands

2. Basic properties of all types of e-m radiation

• Travels (in vacuum) at the speed of light -- FAST!
• Behaves both as waves and indivisible particles (called photons)
• Shorter wavelength implies higher frequency and higher photon energy

3. Spectrum of e-m radiation

• Wien's Law: hotter objects radiate shorter wavelengths
• Stefan-Boltzmann Law: relates radiated luminosity to surface area and temperature
• Three kinds of spectra: continuum, emission line, absorption line

4. Atoms and Radiation

• Atoms emit photons when they are excited by colliding with other atoms in a hot gas

• Each different kind of atom in a gas will emit (or absorb) its own characteristic line spectrum
• Therefore, we can tell what kinds of atoms a gas contains by observing its spectrum

5. Doppler Effect

• When a source of radiation is moving toward (away) from us, the frequency of the radiation that we see is higher (lower) than the frequency that is emitted by the source.
• We can measure the speed of a source of radiation is moving by measuring the frequency (or wavelength) shifts of the spectral lines in its spectrum.

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Last modified January 15, 2001;
Copyright by Richard McCray