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When a laser pointer is pointed at a screen or at a target that spot can be detected because the laser beam falls on the target and bounces back and then is detected by a photosensor.

But, what if it was possible for an electronic eye / photosensor to detect a spot or point of light in just air, without the need of a surface for that spot/point to bounce of. What would be the practical applications of such an invention?

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In order to see something, that "something" must either emit or reflect photons into your eye (or detector). If photons leave a light source in some direction, but are seen/detected "in the air", then the air must have somehow directed those photons toward your eye/detector.

Like this: light

I can only see two ways for this to happen:

  1. Some particles in the air scatter the photons. This happens in many cases.
  2. Magic make the photons change direction. To my knowledge, this has not happened.

Practical implications of the first scenario include

  • guiding the eye toward a star with a laser pointer (photons scatter on particles in the air and thus create a visible ray),

  • studying supernovae looking at the light echo from the light scattering off of surrounding dust particles in their remnants,

  • measuring the size of the broad-line region around supermassive black holes, using the technique known as reverberation mapping (see also this answer),

  • breaking into banks using flour to see laser traps, and, most importantly,

  • summoning Batman.

Practical implications of the second scenario are limited only by your imagination.

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  • $\begingroup$ thank you. No magic to make the photons change direction. they will have to be scattered by air/dust/fog/water vapour/any other aerosols. $\endgroup$ – g g Feb 2 '17 at 7:59
  • $\begingroup$ I am under the impression that light echoes were used for studying supernovae & not the remnants. Also, +1 for the Batman reference. $\endgroup$ – Kyle Kanos Feb 26 '17 at 12:33
  • $\begingroup$ @KyleKanos: Yes, you're right. That's usually the primary goal, but I guess you could say you're studying the remnant at the same time. $\endgroup$ – pela Feb 26 '17 at 13:29
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Light does scatter in air (https://en.wikipedia.org/wiki/Rayleigh_scattering), so one can observe the scattered light and thus detect a light beam without the need for a solid surface. In practice, this is used for remote sensing of the atmosphere. Other possible applications - non-line-of-sight communications (http://iopscience.iop.org/article/10.1088/0268-1242/29/8/084006/meta), eavesdropping of communications that use laser radiation, detection of laser beams in the atmosphere for military purposes.

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