Visible optical beats at the hertz level
  We present a lecture demonstration which produces a visible, beating interference pattern that is the optical analog of demonstrations which produce audible, beating sound-wave interference. The setup is a compact, portable Mach-Zehnder interferometer made of optical components commonly found in laser physics labs. This apparatus may also be built and used in advanced laboratory courses to illustrate concepts in interferometry and laser light modulation. 
Mickey McDonald, Jiyoun Ha, Bart H. McGuyer, Tanya Zelevinsky
physics.optics physics.ed-ph