Doppler effect picture on website: http://www.school-for-champions.com/science/images/waves_doppler_effect.gif
Jim Wiss lecture notes: https://courses.physics.illinois.edu/phys436/p436web/p436web/waves-notes.pdf
More lecture notes: https://courses.physics.illinois.edu/phys436/p436web/p436web/special_rel2-notes.pdf
Doppler effect uses: http://www.exploratorium.edu/hubble/tools/doppler.html
Exoplanets: http://lasp.colorado.edu/education/outerplanets/exoplanets.php