What I remember was a brown or black disc with holes around the perimeter. I remember a lot of holes. This was around 1991 or so.
-----Original Message----- From: time-nuts <[email protected]> On Behalf Of Bruce Griffiths Sent: Saturday, May 12, 2018 4:39 PM To: Discussion of precise time and frequency measurement <[email protected]> Subject: Re: [time-nuts] Anybody have suggestions for time related science fair projects? Like this so called star target?: https://www.edmundoptics.com/test-targets/resolution-test-targets/1-black-1-white-glass-star-target-5deg-wedge-pair-angle/ Bruce > On 13 May 2018 at 02:45 Bob kb8tq <[email protected]> wrote: > > > Hi > > > > > On May 12, 2018, at 7:01 AM, jimlux <[email protected]> wrote: > > > > On 5/11/18 9:08 PM, Jeff Woolsey wrote: > >> David.vanhorn wrote: > >>> Measuring the speed of light (Fizeau or Michelson method? Other > >>> ways) > >>> > >>> > >>> I saw a great demo of this at the Exploratorium in SF. They had a long > >>> spool of fiber optic, a disc with holes, and a light source. When > >>> static, if the light shines through the hole in the disc into the fiber, > >>> then you can see the light coming out the other end of the fiber through > >>> a different hole. When rotating, you increase speed and the fiber > >>> output gets dimmer and dimmer till it's gone. At that point, the light > >>> going into the fiber arrives when the other end is blocked, and vice > >>> versa. High tech, but simple. > >>> > >> My favorite exhibit that we never see anymore. IIRC it was a quarter > >> mile of fiber and a green laser. And ISTR that the disc had one > >> hole on one arm and two radially on the other, but I can't remember > >> why. I thought that the light would pass through the same hole > >> twice, once on the way in and on the way out when that same hole > >> rotated 180 degrees to the other end of the fiber. The disk spun > >> somewhere around 50 rps (60 with an AC motor?). > > > > > > 1km in free space would be 6 microseconds round trip. I'm not sure a disk > > spinning at 3600 rpm would work. you'd need to have the "hole spacing" be > > on the order of 6 microseconds - and at 100 rps (6000 RPM), 10 ms/rev, > > you'd need the sending and receiving hole 6/10000 of a rev apart (about 0.2 > > degrees). > > > > if you had 10 km of fiber, it would be a bit easier. > > I think the term “long fiber” in this case should really be “very very > long”. Exactly how the typical student funds the acquisition of something in > the “many miles” range, I have no idea. > > You could use an optical grating of some sort as your “spinning disk”. > The end of the fiber is going to be mighty small. The spacing on the grating > could be quite tight. Where you get a circular part like that …. > again no idea. > > Bob > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
