You can buy a test spool of 25km of single-mode fiber for $130 on ebay. This is about 125 us of delay, so how can we directly observe this delay? It's easy wi

You can buy a test spool of 25km of single-mode fiber for $130 on ebay. This is about 125 us of delay, so how can we directly observe this delay? It's easy with electronics, but if you could mount visual indicators on a rotating drum the delay would show up as a physical separation: ~1 mm for a .1 meter diameter drum rotating at 2000 RPM..