50 watts!? Dude, a space heater puts out over a 1000 watts. This laser would have to be MUCH more powerful than 50 watts. All that power is going to be dissipated as it heats the atmosphere over several kilometers distance.
Also of note is that a 150 milliwatt green laser (15% of a watt) will shine for miles through the atmosphere.
Not true.
Milliwatt levels can damage the retina.
It depends on atmospheric conditions as to how much beam spreading and attenuation there is.
50 watts of power over a two millimeter diameter area is very powerful. That would badly burn your skin and likely cut through it if it were passed over it slowly.
Yeh, but that space heater is emitting a broad spectrum of light, most of it in the infrared. A laser emits a single frequency (well technically a very narrow range of frequencies), IOW it's all one color of light. It's also coherent, which means that it remains in a small cross sectional area for much longer distances that non coherent and multi frequency light. 50 Watts is a lot of laser power. Nothing compared to the sort of laser used to shoot down missiles, but plenty to damage your eyes. Not much of the energy would go to heating the air, if the frequency was properly selected.
A space heater is not radiating directed energy.
An inexpensive diode LASER rated at one (1) Watt of output (optical) power can pop balloons from across the room, and potentially start fires. An eyeball doesn't stand much of a chance against such a device, even at greater distances.
A much higher powered flash pumped LASER can be fabricated for several hundred dollars. A 30 Watt beam is capable of welding, and punching holes in razor blades/hardened steel. If you know what you are doing, you should be able to rig a 30 Watt gas or ruby (yag) LASER in your garage, on a shoestring budget. Such a homemade LASER could indeed be extremely dangerous.
Atmospheric conditions can play a significant role in diffraction but, by its very nature, a LASER beam is capable of maintaining coherency across vast distances. Even if a 10 Watt beam were 90% diffracted by the time it reached its target, you should still have plenty of coherent photons to damage an eyeball.