Caption: "5 mW 532 nm (i.e., nearly monochromatic green) laser (class IIIb) directed at a palm tree approximately 30 m away. The laser was offset by about 10 cm to the camera's right. Both the camera and laser were mechanically steadied. Photo cropped only." (Slightly edited.)
A laser pointer beam is only visible because of the scattering of some light off some aerosol (e.g., water drops or dust) in the air. In the case of this image, it's water drops in rain or mist.
In general, light rays NOT directed to the observer are invisible.
Credit/Permission: ©
User:Flip619,
2007 /
Creative Commons
CC BY-SA 3.0.
Image link: Wikipedia:
File:Green-lased palm tree (crop).jpg.
Local file: local link: laser_aerosol.html.
File: Optics file:
laser_aerosol.html.