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Given that light is an electromagnetic wave why is it that I can see the ray of light without my eyes being in the path of the ray? For example in this image: enter image description here

The camera is at the side and looking at the light wave and it can detect the light. However if I understand correctly the wave of light is moving left to right in-front of the camera(essentially photons are traveling from left to right) so how can they be detected by the camera. Why doesn't the camera need to be in the path of this ray for detection of light. And if the there are photons moving from each point in the path of the ray towards the camera how is that happening which rays represent that?

Also similar to this why can I see the path/ray of a laser pointer through space when the photons from the laser move from the laser pointer in a straight line and the laser pointer is not directly in-front of my eyes. Do the laser pointer shoot a photon straight which emits an electromagnetic wave at each point in its path that I can see?

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  • $\begingroup$ Does this answer your question? Why do we see laser beams? $\endgroup$
    – John Rennie
    Commented May 31 at 15:31
  • $\begingroup$ The same idea for floating dust near window makes sunlight "visible". $\endgroup$
    – Ng Chung Tak
    Commented May 31 at 17:29

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You are seeing the light scattered from the environment. In the photo above, the "ray" (really probably a vertical "sheet") scatters off the table. Some of that scattered light will reach your eye. Here is a diagram showing how a typical surface (like a table) causes reflected rays to travel in all directions (source):

Diffuse reflection

If the light in the demo in your photo were not a sheet/wide beam but instead a tight low-divergence beam that did not intersect with the table (and the air were clean of dust/other particulates), then you would not be able to see anything from the side.

It is similar with a laser - you can see it because light scatters off of "stuff" in the beam (air, dust, etc.). If it were traveling through the vacuum of space you would not be able to see the beam, because there would be nothing for it to scatter off of. Here is a quick (crude) diagram I made:

Laser beam scattering off of dust

Hopefully this shows better how dust/other particles in the path of the beam lead to light being scattered out of the path of the beam (basically in a random direction), thus enabling it to become visible from the side.

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