Procedure and Tasks

A laser, two lenses with f=52mm and f=250mm, a half reflecting mirror, a set of polarizers and the rotating mirror are already mounted onto an optical bench and aligned. The principal arrangement is shown in the setup figure. The fixed mirror MF has a radius of 13.5m. In our lab the path between the rotating mirror and the spherical mirror is folded back by an additional flat fixed mirror.

  1. Measure the path length between rotating mirror, flat mirror and spherical mirror.
  2. Unplug the power supply of the rotating mirror. Align the rotating mirror so that the reflected beam hits the flat mirror. One aligned, tighten the lock-screw on the rotating mirror to keep it stationary. DO NOT OVERTIGHTEN!
  3. Align the flat mirror so that the reflected beam hits the spherical mirror.
  4. Align the spherical mirror so that the reflected beam returns to the rotating mirror.
  5. Find the focal point of the beam after it is finally reflected by the beam splitter.
  6. Reduce the light intensity with the two polarizers and bring the photo sensitive layer of the SBIG-camera (without lenses) into the position of the focal point.
  7. Start the software (see below).
  8. Release the lock-screw of the rotating mirror, plug in the cable of its power supply and start the rotation. Use frequencies of 250, 500, 750 and 1000 Hz in the positive and negative direction.
  9. For each frequency setting determine the position of the beam on the screen (in units of pixel, 1 pixel =7.4 mm)
  10. Plot the position as a function of rotational frequency, determine the change of angle Da with respect to the rotating mirror and determine the velocity of light.



  1. Before the computer is turned on, connect the cable from the SBIG-box with the computer (replace the printer cable).
  2. Turn on the SBIG box, turn on the computer.
  3. Load "CCDOPS"
  4. Goto Camera, -> Setup, temperature regulation->active
  5. Goto Camera, -> Grab
  6. Goto Display, -> Show Crosshair, choose for example 25x25.
  7. Determine the position of the image.
  8. Store image (for example Ccw750 for a counterclockwise rotation with f=750Hz)
  9. Print one of the images.
  10. When finished with the measurements turn of the temperature regulation: Goto Camera, -> Setup, temperature regulation->off, wait until the temperature reaches room temperature before you turn of the power for the camera.
  11. Turn off the SBIG box.