stroboscope (n.) Look up stroboscope at Dictionary.com
"instrument for studying motion by periodic light," 1896, from -scope + Greek strobos "a twisting, act of whirling," from PIE *streb(h)- "to wind, turn" (see strophe). Earlier as the name of a similar device used as a "scientific toy" [OED]. Related: Stroboscopic (1846).