# What kind of “camera” and “light” source should I use to detect the path along which the light moves?

I would like to have an image ( in any kind of space ), where I see the path of a "light" source. In my understanding the most common, directed source would be a laser pointer.

a. Is this correct? If not, what would be an other option?

As far as I know, to be able to "see" the path of the light, the material through the light passes should reflect some of it into my "camera", like smoke.

b. Is this correct? Do I always need this "material"? Does the air reflect any of it?

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 Are you saying you want to see the path along which the light source moves, or the path along which the light itself moves? – David Zaslavsky♦ Oct 15 '12 at 19:51 Second one ( the path along which the light moves ), modified it accordingly. – burninggramma Oct 15 '12 at 19:55 It is not possible to see light move you can only see light being interacted with I.e. reflected or absorbed. – Argus Oct 16 '12 at 1:32 Take a look on the question after point a. – burninggramma Oct 16 '12 at 7:39

Question 1

A Laser gives light with least amount of angular spread. As far as my knowledge goes. Though a Directed pointer can be made out of any light source.

Question 2

The air molecules will scatter off some light, But this may not be noticeable unless you use extremely sensitive detection apparatus. I dont have any quantitative estimates of how sensitive and so on.

To me it seems absolutely essential that one need to place this "material" such as smoke, or dust particle in the path of the light to measure it. There is a simple reason for this. In vacuum, If you are interested in determining if light passes through a certain region or not, You have to place something there, Or the existence of light in that region cannot be detected. The very placing of this material, will however disturb the behaviour of the light beam.

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Light can only be detected if some of the photons in the light beam fall on a detector. It is not possible to "see" a photon flying past, in the way we can see a plane flying past. A photon is light, it does not deflect light. As photons always travel in a straight line (a geodesic, to be exact), a light beam can only be detected under 2 circumstances:

1. The detector is in the beam of light. This is easy if your detector completely surrounds the light source. For example, put a sphere of translucent glass around the source, and the beam will show as a spot on the surface. Of course, this does not show the complete path, only the spot where it hits the detector. By moving a detector around you can reconstruct the entire path of the beam - but of course, as light travels in a straight line, all you need to know is the locations of the source and of 1 point away from the source.

2. Some of the light from the beam is scattered to the detector. For this to happen some material must be in the path of the light. This makes it impossible to detect a beam in a complete vacuum. Any material will scatter some of the light and the sensitivity of your detector will determine which materials are suitable. Smoke is often used for this, but remember that any scattering affects the light beam.

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While it is correct that you can not observe the photons directly without disturbing them it is possible to take photographs of "light" to detect its path. In principle you only need a very fast camera.

MIT has developed such a system and the results are interesting. You can even see around corners with it. It is a bit difficult to explain but the video on their website shows the effect quite nicely: Visualizing photons.

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