"If I have seen further it is by standing on the shoulders of giants."
Isaac Newton
To many people, it appears as though Einstein made his discoveries in an intellectual vacuum. This is not true. One experiment which greatly assisted Einstein in his discoveries was the Michelson-Morley experiment.
Michelson and Morley designed an experiment that demonstrates the constant nature of the speed of light. They reflected light beams between two mirrors in both the parallel and perpendicular planes to the earth’s orbit and found no detectable difference in the measured speed of light (light has a constant speed).
Below is a similar experiment that helps illustrate light as constant.
Create two identical clocks that reflect light between two mirrors so that each cycle of there and back takes one second. If you then set one of the clocks in motion in a direction perpendicular to the reflected light, the beam will now have further to travel to complete the cycle, and therefore the counter will advance slower than the stationary clock.
