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Can a highspeed camera show a laserbeam moving?

  1. Jun 10, 2010 #1
    Just wondering if we could make a high speed camera that shows a actually laser beam slowly moving across the room in front of the camera? Given its moving at c, is there a camera that can record fast enough to see it move at walking speed? Like slow mo vids of a bullet flying past the camera.
     
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  3. Jun 10, 2010 #2

    Pengwuino

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    No, and I think there has been a thread about this for that basically pointed out "What do you think a camera actually detects?"
     
  4. Jun 11, 2010 #3
    Light is invisible because it makes things visible. :D
     
  5. Jun 11, 2010 #4
    Theoretically yes, if you can capture fast enough you'll be able to see a laser beam moving across the screen.

    In reality, no. We simply can't make something that would take pictures fast enough. A 1000 fps camera is fairly high speed. In the time the next frame comes round light has moved about 300 thousand meters.

    The fastest high speed camera availalbe is about 25 million fps according to wiki, (but I don't think it's a camera in the conventional sense, you wont get visual images just a line probably). In this time light will have travelled 11m. Depending on the scale of the camera, you'll be able to see that moving.
     
  6. Jun 11, 2010 #5

    Vanadium 50

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    This could, however, be faked with a pulsed laser in sync with the frame rate of the camera.
     
  7. Jun 11, 2010 #6
    Well, that depends on how you define "room" :wink:

    With a room consisting of air, where light is moving close to c it isn't possible, but it has been demonstrated in both crystals and certain gases, that it's possible to prepare materials in such a way (with a very strong and specific dispersion profile), that the speed of light is reduced to very low quantities, even down to meters per second.

    This can even be prepared such that the speed is so slow only in one direction of the crystal/gas but normal in the other directions, which means that any photon scattering events that occur inside the material that makes the photon go in another direction, would leave the crystal very fast and could thus be detected by a more normal camera. With this setup it could be possible to see the light moving even with your eyes, although it's also cheating a bit of course :tongue2:
     
  8. Jun 11, 2010 #7
    Hold on! First of all can a light beam be photographed? Doesn't make sense to me.
     
  9. Jun 11, 2010 #8
    You can see lasers, depending on the frequency. You'd need say a smoke room to see it visually. (I just assumed he was talkign about a room where lasers can be seen). EDIT: Although it's technically not then a laser you are seeing it's the reflected light.

    However to 'see' the beam moving you'd need something that works of photon detection. At least thats how the fast fps cameras work as far as I know, and it's why you only get a line, not a proper image.
     
  10. Jun 11, 2010 #9

    Cthugha

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    I suppose the closest approach to taking a video recording of moving light can be realized by choosing an approach similar to what xxChrisxx suggested:
    Take a short laser beam and manage somehow to get some of the light reflected at an well defined angle over a length of 3 mm or so and then shine this line of 3 mm length on the entrance slit of a streak camera. Inside the camera these photons will hit a photocathode and be converted to photoelectrons. Now you can accelerate them towards some screen which will glow when hit by electrons (usually some phosphor screen) and at the same time apply some sinusoidal or ramped vertical deflection voltage, so that the position where an electron hits the phosphor screen will depend on the time when it hit the photocathode. Now just take a picture of the screen afterglow.
    You will get a picture that shows the position where the light hit the photocathode (corresponding to the position where it was when it was reflected) on the horizontal axis and that shows the time when the photon arrived at the photocathode on the vertical axis. So if you do it correctly, you should get a diagonal line moving from the top left to the bottom right, correlating arrival position and time. The possible timing resolution in streak cameras is on the order of 500 fs to 1.5 ps. Typical photocathode sizes are about 3 mm (corresponding to roughly 10 ps travel time at the speed of light), so it should be possible to monitor the movement of a short light pulse. However, this is of course just monitoring, not taking a direct picture.
     
  11. Jun 11, 2010 #10
    thanks for the replies.

    What about a large atmosphere at night like a few light seconds across in length with the camera showing the line of laser racing across the whole scene? I know you dont see the actual photons that are moving in the forward direction, but isnt it the same as a bullet, you see the light coming off it. So should you see a line just move by quite fast? When i shoot my green laser pointer at the sky it just appears. Be great to see it extend/rise into the sky.
     
  12. Jun 11, 2010 #11
    Ah, well no you won't be able to see that.
     
  13. Jun 11, 2010 #12
    dam. What if the beam was from the Deathstar??? lol
     
  14. Jun 11, 2010 #13

    russ_watters

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    The atmosphere is much too small for that - a few light seconds takes you to the moon.
    The beam fromthe death star would only be visible to you if you were in front of it...
     
  15. Jun 11, 2010 #14
    yeah I was just imagining a large atmosphere or something long with gas in it to show the beam. And it would scatter so why wouldn't you see it?
     
  16. Jun 11, 2010 #15

    Andy Resnick

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    Last edited by a moderator: May 4, 2017
  17. Jun 11, 2010 #16
    For all the people questioning how we would 'see' the beam, I'm assuming it was implied that the room was filled with smoke or fog or something like that that would scatter the beam, and that the camera was perpendicular to the beam.
     
  18. Jun 11, 2010 #17

    Borek

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    I wonder - during cosmic scale explosions, radiation going out can be scattered by the matter if any happens to be around. We tend to observe such events even for many years, so I wouldn't be surprised if we already have "traveling light" on tape.
     
  19. Jun 11, 2010 #18

    Integral

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    Can you see a laser beam?

    greenbeam.jpg

    Obviously.

    The laser in on the right, to the left is a power meter, the intense green circle of light on the laser is reflected from the power meter. In between the laser and the power meter is a PIN diode used to measure pulse width from light reflected off of the power meter.

    This is 30W Coherent laser.
     
  20. Jun 12, 2010 #19

    Cthugha

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    Sure, this is trivial. But the question was whether one can see it moving.
     
  21. Jun 12, 2010 #20

    Redbelly98

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    A rough calculation of the required camera speed is trivial.

    Divide a normal sized room into, say, three time increments, i.e. the beam is to travel 15 to 20 feet in three frame increments. This is equivalent to the beam traveling about 2 meters in each frame's time increment:
    c Δt = 2 m
    Δt = 2m / c = 2m / (0.3 m/ns) ≈ 0.7 ns​
    So the question becomes, are there cameras that are faster than 1 ns per frame? Googling fastest camera suggests that 6 million frames per second, or 17 ns, is the current record. The light beam will travel 50 meters per frame, so this won't be visible in a normal sized room. But outside in a football-stadium-sized area, on a foggy night, it looks like one could see this effect.
     
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