We must first determine what we mean by the problem. Because light will not stop spreading. The more interesting question is how far we can see the reflection of the laser under ideal conditions. In complete darkness, when it shines on a white surface, we can see a <5mW 650nm laser from 500-1 km away. The divergence will make the projected point 1km away become a green laser pointer with a diameter of 20-30m, and it is very weak.
If it is a real laser with zero divergence, and there is no medium to attenuate or diverge it, it should have an infinite range. However, the earth’s atmosphere does attenuate, and more importantly, it diverges the beam, so this is unrealistic. Even if you can create a laser that is strong enough to penetrate the earth’s atmosphere, it will still radiate when it escapes the atmosphere. As the beam further enters space, this will gradually reduce the intensity of the light, causing it to disappear into the background radiation after a certain distance.
Depends on the power… Regulatory laser pointer with Class II laser (visible light less than 5mW) can be seen in ordinary air, assuming the normal divergence of the laser diode can be seen within a few hundred meters…. I bought an unsupervised laser device with a Class IIIb laser (60mW green) on the Chinese market, which can pass through 2 kilometers of fog…
By the way, this kind of thing will be classified as a weapon when it enters the United States and gets caught. You heard the truth. It will keep walking, keep walking, until it hits something. It will also hit something during the journey. Air is a thing. Dust is a thing. Fluff, water vapor, pollen, flying insects, dust, smoke essence, all things. The photon beam pouring out of the laser pointer object will hit something sooner or later, several at a time, until the last one hits its doom. There is no more laser beam.
The divergence of laser beams is usually much smaller than other light sources, but still small because their design has been optimized to the basic limit set by the initial aperture width. In principle, an infinite plane wave will not “diverge” because it is composed of plane wave fronts that always move steadily in the same direction, perpendicular to their own plane. (Of course, it can also be infinitely wide, which is not very helpful if you want to project a point on a specific target.) Sadly, if you do cut an infinitely wide wave into a beam of finite width, It will inevitably spread out gradually on the edge.