In 1926, american physicist albert michelson used to measure the speed of light.

Albert Michelson had many attempts to measure the speed of light during his career, yet his self-acclaimed definitive measurements were achieved using an 8-sided mirror that rotated around an axis at the speed of 528 revolutions per second. He used a large distance between these reflecting mirrors on a mountain-site, and they were approximately 35.5 km apart. In order to determine what the speed of light was, he would calculate in meters per second how long it would take the light to reach the one reflector and travel back compared to the time it took for one of the eight mirrors to complete a 360 degree rotation.

Initial velocity,  u = 4.9 m/sfinal velocity = v m/sinitial height,  h₁ =  245 m    final height, h₂ = 0a = -9.8  m/s²time taken = t secondss = ut + 0.5at²⇒  (h₂-h₁) = ut + 0.5at²⇒  0-245 = 4.9t + 0.5×(-9.8)×t²⇒ -245 = 4.9t - 4.9t²⇒ 4.9t² -4.9t -245 =0solving it, we get   t = 7.59sv  = u +  at = 4.9 -9.8×7.59 = 4.9 - 74.38 = -69.48 m/svelocity is  69.48 m/s downward