# How do you measure the speed of light?

**URL:** <https://boards.straightdope.com/t/how-do-you-measure-the-speed-of-light/19216>\
**Category:** Factual Questions\
**Created:** [May 11, 2000, 10:56am UTC](https://boards.straightdope.com/t/how-do-you-measure-the-speed-of-light/19216 "2000-05-11T10:56:56Z")\
**Posts on this page:** 1\
**Page:** 2

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**Author:** ![december](https://avatars.discourse-cdn.com/v4/letter/d/838e76/32.png) [@december](https://boards.straightdope.com/u/december)\
**Post date:** [May 12, 2000, 5:17pm UTC](https://boards.straightdope.com/t/how-do-you-measure-the-speed-of-light/19216/21 "2000-05-12T17:17:29Z")

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In 1961, my U. of Chicago Physics prof showed us an actual device used by M&M to measure the S of L. As I recall it had two mirrors facing each other (or maybe a light source and a mirror.) One or both of the mirrors was rotating rapidly.

I remember being surprised at how small it was. As I recall, the two mirrors were only separated by a couple of feet. Because of the rapid rotation, the time taken for the light beam to make the round trip to the rotating mirror and back could be measured by a noticeable distance between the source of the light and its reflection back.

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