Laser pointers manufacturers claim that some pointers have range of several kilometers.
Okay, they use a powerful laser, but that powerful laser usually has power less than one watt. Okay, the laser beam is very focused.
But what about atmosphere particles like dust and vapor? Why don't these particles diffuse the beam and make it lose energy?
How does it happen that a less than one watt laser can have several kilometers range in atmosphere?
Answer
A laser pointer's energy and lights are concentrated in a very small light cone to reach a quite high intensity. So its labelled Wattage is much smaller than a usual bulb while its light is very strong at one point. And the labled power on laser pointers is the output power. The typical input power of a 50mW pointer is about 0.5 W. This make the difference looks bigger.
Actually common laser pointers used by teachers and lecturers are less than 5mW. The farthest distance to see their lights is about dozens of meters. But the laser pointers used by professional and amateur astronomers are much more powerful. You can see the light path caused by Tyndall effect directly as below image(source, though this impressive light path is not generated by a handheld pointer, the scence is similar.)
The energy reduction is mainly caused by Rayleigh scattering, which is relatively small compared with laser's intensity. So the beam can easily reach several miles away with the power higher than 50 mW.
Update: Georg insist the previous image can not be used to discribe laser pointers. So I add another real pointer picture here. But I have no feeling about distance with this one.
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