  A mechanical beam chopper consists of a rotating disc of regularly spaced
wide slits which allow light to pass through them. A continuous light beam,
after passing through the rotating disc, is switched-on and switched-off
periodically, and a series of optical pulses are produced. The intensity of
each pulse is expected to rise and fall smoothly with time. However, a careful
study has revealed that the edges of mechanically chopped laser light pulses
consist of periodic intensity undulations which can be detected with a photo
detector. It has been shown in this paper that the intensity undulations in
mechanically chopped laser pulses are produced by diffraction of light from the
rotating disc and a detailed explanation of the intensity undulations is given.
The experiment provides an efficient method to capture a one dimensional
diffraction profile of light from a straight sharp-edge in the time domain. In
addition, the experiment accurately measured wavelengths of three different
laser beams from the undulations in mechanically chopped laser light pulses.
