If you pointed a flash light at two mirrors, would the light stop?
If you pointed a flash light at a mirror aimed at another mirror, then turned of the flash light, would the light just continue to bounce back and forth between the 2 mirrors?
The mirrors are NOT 100% reflective. Because of the immense speed of the light photons involved, it would seem to stop almost instantly. However this method was used centuries ago to try to measure the speed of light by Galileo and later by Fizeau and Foucault.
The idea is to have two people far away from each other, with covered lanterns. One uncovers his lantern, then the other immediately uncovers his on seeing the light from the first. This routine is to be practised with the two close together, so they will get used to the reaction times involved, then they are to do it two or three miles apart, or even further using telescopes, to see if the time interval is perceptibly lengthened. Galileo claims he actually tried the experiment at distances less than a mile, and couldn’t detect a time lag. From this one can certainly deduce that light travels at least ten times faster than sound.
We can see, though, that if the two lanterns are ten miles apart, the time lag is of order one-ten thousandth of a second, and it is difficult to see how to arrange that. This technical problem was solved in France about 1850 by two rivals, Fizeau and Foucault, using slightly different techniques. In Fizeau’s apparatus, a beam of light shone between the teeth of a rapidly rotating toothed wheel, so the “lantern” was constantly being covered and uncovered. Instead of a second lantern far away, Fizeau simply had a mirror, reflecting the beam back, where it passed a second time between the teeth of the wheel. The idea was, the blip of light that went out through one gap between teeth would only make it back through the same gap if the teeth had not had time to move over significantly during the round trip time to the far away mirror. It was not difficult to make a wheel with a hundred teeth, and to rotate it hundreds of times a second, so the time for a tooth to move over could be arranged to be a fraction of one ten thousandth of a second. The method worked. Foucault’s method was based on the same general idea, but instead of a toothed wheel, he shone the beam on to a rotating mirror. At one point in the mirror’s rotation, the reflected beam fell on a distant mirror, which reflected it right back to the rotating mirror, which meanwhile had turned through a small angle. After this second reflection from the rotating mirror, the position of the beam was carefully measured. This made it possible to figure out how far the mirror had turned during the time it took the light to make the round trip to the distant mirror, and since the rate of rotation of the mirror was known, the speed of light could be figured out. These techniques gave the speed of light with an accuracy of about 1,000 miles per second.
See source below
Advanced glint shine light effect Flash tutorial – Intermediate mask technique
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Tags: beam of light, faster than sound, flash light, light, light flash, second time, seeing the light, speed of light, time lag, toothed wheel
