# [time-nuts] Anybody have suggestions for time related science fair projects?

Bob kb8tq kb8tq at n1k.org
Sat May 12 10:45:55 EDT 2018

```Hi

> On May 12, 2018, at 7:01 AM, jimlux <jimlux at earthlink.net> 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

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