# Laser + parallel mirrors = see light travel? **URL:** \ **Category:** Factual Questions\ **Created:** [July 4, 2012, 4:47pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225 "2012-07-04T16:47:55Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [July 5, 2012, 7:23am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/21 "2012-07-05T07:23:27Z")
Hmm but you want some scattering, otherwise you won’t be able to see the beam. Maybe I could use a stronger laser. 1 W handheld laser pointers are available now, maybe [someone](http://boards.straightdope.com/sdmb/showthread.php?t=637533) with one could try. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 5, 2012, 7:38am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/22 "2012-07-05T07:38:08Z")
> [@AaronX](#): > > Hmm but you want some scattering, otherwise you won’t be able to see the beam. Maybe I could use a stronger laser. 1 W handheld laser pointers are available now, maybe [someone](http://boards.straightdope.com/sdmb/showthread.php?t=637533) with one could try. Remember that the beam will be folded over itself many times. A single beam at 1 atm will look no brighter than a beam bounced 1000 times over itself at 0.001 atm. 1 W wouldn’t be nearly enough for 3e7 meters at 1 atm. There are probably some other atmospheric factors that would come into play as well, making it even worse. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [July 13, 2012, 7:04am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/23 "2012-07-13T07:04:16Z")
Ok I ran this through my physics intuition and I think what will happen is the fading of the light is slower than its traveling. i.e. when you turn on the laser, you will get an instant (to your perception) gradient of laser beams, not see them gradually appear. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 13, 2012, 12:11pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/24 "2012-07-13T12:11:18Z")
> [@John\_DiFool](#): > > Apophrycal story: I read an account of a star party once, and someone decided to shoot their flash through the base of one of the big telescopes. People standing nearby said that the flash looked like a “photon torpedo” as it shot into the sky. :dubious: Atmospheric scattering would send some light from the departing pulse back to the observers on the ground - but the light pulse would be through the ~100-mile-thick atmosphere in about 1/2 a millisecond, after which backscatter (from orbital/space dust) would be miniminal; I would think it would be difficult to perceive the motion within such a short time period. > [@CalMeacham](#): > > Lasers, by the way, don’t often use two perfectly parallel flat mirrors – it’s too easy for the beam to “walk off”. It’s more comon to have one or both mirrors concave, and to place them an appropriate distance apart, so that a beam that tries to “walk off” will be re-=directed back towards the axis. In grad school I had a copper vapor laser that could be fitted with either type of optics. It arrived with flat mirrors at either end of the chamber, but we purchased concave optics that resulted in a far less divergent (more parallel) output beam. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [July 13, 2012, 1:17pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/25 "2012-07-13T13:17:26Z")
> [@Machine\_Elf](#): > > :dubious: > > Atmospheric scattering would send some light from the departing pulse back to the observers on the ground - but the light pulse would be through the ~100-mile-thick atmosphere in about 1/2 a millisecond, after which backscatter (from orbital/space dust) would be miniminal; I would think it would be difficult to perceive the motion within such a short time period. > > In grad school I had a copper vapor laser that could be fitted with either type of optics. It arrived with flat mirrors at either end of the chamber, but we purchased concave optics that resulted in a far less divergent (more parallel) output beam. Flat mirrors aren’t unheard of. Old Nd:YAG lasers used flat-flat mirrors because the neodymium YAG rod heated up and acted like an internal lens, so you had an effective stable cavity. Some nitrogen lasers used flat mirrors because they were more of glorified ASE (Amplified Spontaneous Emission) devices than stable resonator cavities, and the beam only had to rattle back and forth a few times before it left the cavity anyway. But, in general, flat-flat cavities are avoided because they’re unstable – a slight tilt makes the beam walk off the axis. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [February 22, 2013, 3:33pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/26 "2013-02-22T15:33:46Z")
No, it’d be like using stride length to guess your height. Or shoe size to guess… nevermind Ok I was replying to some posts that seem to have disappeared. Or is it just I can’t see them? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 22, 2013, 3:48pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/27 "2013-02-22T15:48:34Z")
> [@CalMeacham](#): > > If your mirrors reflect 99.99% of the light, that’s approximately 10,000 bounces. If your mirrors are about 4" apart (10 vcm), you can expect an infinitesimally fast burst of light to be absorbed or scattered in about 0.3 milliseconds. We have a [500-meter vacuum chamber](http://optics.nasa.gov/facilities/xraycal.html) at NASA Marshall Space Flight Center. So if we put flat mirrors at opposite ends, it should scale up to 1.5 seconds. However, with mirrors so far apart, there will be additional loss and widening of the beam due to diffraction; I’m sure these factors will become dominant long before 10,000 bounces. (Unless the mirrors are _very_ large - but large flat mirrors are _very_ expensive. ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [February 22, 2013, 4:57pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/28 "2013-02-22T16:57:38Z")
> [@AaronX](#): > > No, it’d be like using stride length to guess your height. Or shoe size to guess.. nevermind > > Ok I was replying to some posts that seem to have disappeared. Or is it just I can’t see them? They could have been bumped by a spammer and disappeared by a mod. That’s been happening lately. ---
**Author:** ![jharvey963](https://avatars.discourse-cdn.com/v4/letter/j/54ee81/32.png) [@jharvey963](https://boards.straightdope.com/u/jharvey963)\ **Post date:** [February 22, 2013, 5:58pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/29 "2013-02-22T17:58:48Z")
Actually, there was just a video of ULTRA high speed video of [light moving through a coke bottle](http://www.youtube.com/watch?v=nkbr-FItV00). Pretty impressive. J. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [February 23, 2013, 6:08am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/30 "2013-02-23T06:08:03Z")
> [@jharvey963](#): > > Actually, there was just a video of ULTRA high speed video of [light moving through a coke bottle](http://www.youtube.com/watch?v=nkbr-FItV00). > > Pretty impressive. > > J. Hey, that’s something like what I was trying to do. Why did they use a pulse? If they had used a beam, we’d just see a beam grow, right? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 23, 2013, 7:54am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/31 "2013-02-23T07:54:46Z")
> [@AaronX](#): > > Ok I was replying to some posts that seem to have disappeared. Or is it just I can’t see them? > [@cochrane](#): > > They could have been bumped by a spammer and disappeared by a mod. That’s been happening lately. Someone bumped the thread to ask a new, not-exactly related question and a mod spun it off as a new thread - here: > **[Laser Beam Travel](https://boards.straightdope.com/sdmb/showthread.php?t=683177)** > > i m working on a project where i m using laser as transmitter and IR as receiver. when laser beam falls on IR receiver it gives output which goes to controller and performs others tasks. i m using many IR receiver in matrix form and all are closed to... ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 23, 2013, 7:59am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/32 "2013-02-23T07:59:47Z")
> [@jharvey963](#): > > Actually, there was just a video of ULTRA high speed video of [light moving through a coke bottle](http://www.youtube.com/watch?v=nkbr-FItV00). > > Pretty impressive. > > J. That looks really fake. How did they create a bright, visible beam of white light that travels as a pulse/packet just a few centimetres long? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 23, 2013, 9:56am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/33 "2013-02-23T09:56:17Z")
> [@Mangetout](#): > > That looks really fake. How did they create a bright, visible beam of white light that travels as a pulse/packet just a few centimetres long? It’s real, but it’s a bit of a cheat. They have a laser that can emit very short pulses (femtoseconds), and a camera with an exposure time of the same. They shoot a laser pulse, wait X femtoseconds, and then snap a picture. Then, they repeat the process, but wait X+1 femtoseconds before snapping a picture. Then wait X+2 femtoseconds. Repeat hundreds of times and you get a video. Except that it’s worse than that. The exposure time is so short that the camera can’t collect enough light in one shot. So for each frame, they have to repeat it hundreds of times before they can collect enough light for a low-noise image. Except that it’s worse than that. Their camera is only one row of pixels, so they have to repeat it for each scanline of the image. And of course, it’s worse than that. The sensor is monochrome, so they have to repeat _that_ process for each primary color. So the whole thing takes a very long time. But it is equivalent to what you’d see if you did have a camera that fast, so in that sense it’s real. It’s certainly not computer generated or anything. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 23, 2013, 10:25am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/34 "2013-02-23T10:25:41Z")
So what is the spatial length of a pulse of light with a few femtoseconds of duration. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [February 23, 2013, 11:20am UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/35 "2013-02-23T11:20:06Z")
> [@Dr.Strangelove](#): > > So the whole thing takes a very long time. But it is equivalent to what you’d see if you did have a camera that fast, so in that sense it’s real. It’s certainly not computer generated or anything. So it’s only usable on highly repeatable processes. Useless on explosions and such. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [February 23, 2013, 4:51pm UTC](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225/36 "2013-02-23T16:51:50Z")
> [@Mangetout](#): > > So what is the spatial length of a pulse of light with a few femtoseconds of duration. About [300 nanometers per femtosecond](http://www.wolframalpha.com/input/?i=speed+of+light+*+femtosecond) (300,000,000 m/s \* 1 fs), which is actually LESS than the wavelength of light unless you go down to UV, so the pulse is probably more like 5-10 femtoseconds at least. Actually, looking at that video, it seems more like 10-100 picoseconds or so (3-30 mm long; of course, the actual exposure time is femtoseconds, which makes sense if you want minimum motion blur). [Previous page](https://boards.straightdope.com/t/laser-parallel-mirrors-see-light-travel/627225.md?page=1)