Video Documentary film on the Future of Space Colonisation by the Human Race.
It is probable that the first space colonists in Earths Future History have already been born....
For our demonstration, we used a He-Ne laser as the source (a much better source than Foucault had!). At these distances, the laser acts essentially as a point source of light, so it is necessary to focus the beam so that the points of focus are the fixed mirror (M) and source and observation point (S). Let D’ be the distance between the rotating mirror and the source/observation point, G be the distance between the lens (L) and fixed mirror (M) and G’ be the distance the light travels from the lens to the source point (LR+RS). Then the lens makers’ equation tells us that the focal length, f, of the lens must be:
For our setup, we used a lens with a focal length of 5m, and set G and G’ to be 10m. We set the distance D to be approximately 13m and D’ to be 7m. In order to observe the deflection of the beam, we inserted a thin (to minimize internal reflections) microscope slide beam splitter just before the source. Our rotating mirror is based on a Bosch router motor, with a peak speed of 27000rpm at 120V AC.
Alignment is slightly tricky. I suggest establishing the beam to the rotating mirror first, and then moving the rotating mirror to point approximately at the desired location of the fixed mirror. The beam should go to the center of the lens and the fixed mirror, and then (harder) the spot returning from the fixed mirror must be aligned on the rotating mirror. At this point, the returned beam should be visible after the beam splitter! Chalk dust and water mist make great diagnostics for finding the beam and small variations in the lens position sometimes can compensate for small mirror misalignments. This takes some patience and good clamps for the lens and fixed mirrors!
One you have established a view of the beam after the beam splitter, I suggest measuring the beam spot position at high and low frequency. The deviation of the beam spot, δx, will be