<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Michelson_interferometer" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Michelson_interferometer</a><p>Basically fire two lasers at right angles to each other at mirrors, and see the pattern when they bounce back.