Capturing the speed of sound and speed of light on video.

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Discussion Overview

The discussion revolves around the concepts of capturing sound and light phenomena using high-speed cameras, specifically focusing on the relationship between shockwaves from explosions and sound perception, as well as the potential for visualizing light beams in motion. The scope includes theoretical considerations and practical applications in experimental setups.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that people in the distance would not hear the sound of an explosion until the visible shockwave reaches them, questioning whether the pressure wave captured in video is the same as the sound waves heard.
  • Others argue that while the shockwave can be seen, the sound is perceived later due to the difference in propagation speeds, with examples of ground rumbling preceding sound perception provided.
  • There is a discussion about whether a camera could be developed to record frames faster than the speed of light, with some participants expressing skepticism about the feasibility of such technology.
  • Some participants suggest that it might be theoretically possible to visualize a light beam under specific conditions, such as using a setup perpendicular to a laser to observe the beam extending from its source.
  • One participant mentions that human reaction time could affect the perception of light, indicating that a significant distance would be required to observe the effect of light travel time on human perception.
  • A reference to light echoes in astronomy is made, suggesting a parallel phenomenon that could relate to the discussion on light perception.
  • Streak cameras are mentioned as a technology that could potentially track short pulses of light, with links provided for further information.

Areas of Agreement / Disagreement

Participants express varying views on the feasibility of capturing light in motion and the relationship between shockwaves and sound perception. No consensus is reached on the possibility of recording light beams or the implications of human perception related to light travel time.

Contextual Notes

Limitations include the dependence on specific experimental setups and the need for significant distances to observe effects related to light travel time. The discussion also highlights the complexity of sound and light propagation, which may not be fully resolved in the current context.

n0sferatu
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Hi!

Consider a high-speed camera set up to record an explosion and the resulting shockwave emanating from the explosion. In the same frame you can see the shock wave as well as people in the distance. Is it true that the people in the distance wouldn't hear the sound of the explosion until the visible wave in the video hits their eardrums? That is, is the pressure wave that emanates from the explosion and that is captured in the video the same as the sound waves that the person hears?

Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes? Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?

Just curious! Thanks!
 
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n0sferatu said:
Hi!
Welcome to PF!
Consider a high-speed camera set up to record an explosion and the resulting shockwave emanating from the explosion. In the same frame you can see the shock wave as well as people in the distance. Is it true that the people in the distance wouldn't hear the sound of the explosion until the visible wave in the video hits their eardrums? That is, is the pressure wave that emanates from the explosion and that is captured in the video the same as the sound waves that the person hears?
Correct. Example:

A few tidbits about this:
1. You can see the shock wave in air kicking up dust as it races across the desert.
2. The speed of sound in Earth is much higher than in air, so before the air shockwave and sound hit you, the ground starts rumbling under your feet.
3. For dramatic effect, the TV station synced the sound to the explosion. What actually happens is you hear nothing when you first see the explosion, then the ground starts to shake under you, then the shock wave hits you. I recently saw a Mythbusters clip where they describe this - I'll have to look for it.
Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels?
That doesn't make sense. Cameras record in frames per second - that's a rate, but not a speed. Speed is meters per second.
If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes?
The only light you can see is light that hits your eyes. It is however, theoretically possible to set up perpendicular to a laser and watch the beam extend from its source when turned-on. The beam is visible because dust in the air reflects some of the light away from the path of the laser and into the camer/your eyes.
Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?
With the setup I described above, you could witness something like that. Note, however, you'd need a baseline of a hundred thousand km for the experiment to show that effect on a human timescale. Human reaction time is on the order of 1/10 of a second.
 
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n0sferatu said:
Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes? Also, if you stopped at a frame in the middle of the footage (with the light beam 1/2 way from the source to the persons eyes), would the person not realize that the flash light was on at that exact moment because the photons of the light beam hadn't hit their retinas yet?

Pretty much this exact phenomenon is observed in astronomy: http://en.wikipedia.org/wiki/Light_echo
 
n0sferatu said:
Next, consider the same setup except using a flash light that is aimed at a person's eyes. Could there ever be a camera developed that can record frames faster than the speed that light travels? If so, could you then play back the video and actually see the beam of light traveling in slow-motion through space towards the persons eyes?

AFAIK, streak cameras are used (essentially) for this purpose- they are used for tracking short pulses.

http://www.optronis.com/support/streak-kameras.html
http://sales.hamamatsu.com/en/products/system-division/ultra-fast/streak-systems.php
http://sales.hamamatsu.com/assets/pdf/catsandguides/e_streakh.pdf
 
Thanks for the replies and very interesting links, I appreciate it!
 

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