Humans have successfully built lasers from microwave frequencies to x-ray frequencies. Some of frequency ranges wouldn't penetrate the atmosphere (e.g. ultraviolet-x-ray). Other specific frequencies (e.g. 9-13 $\mu$ wavelength for $CO_2$) are absorbed by the atmosphere's constituent molecules.
Under ideal conditions for the laser, the beam won't be absorbed or reflected by the atmosphere. That means the only thing you'd see is a literally blindingly bright spot at the target.
If a human with unprotected vision looks at the target during the shot, there is a good chance they will be permanently blinded. If a human with unprotected vision is in LOS but not looking, there's still a chance they'll be blinded by reflected light. The effects of the atmospheric plasma will be exactly like lightning (sight & sound). The stroke will be perfectly straight and visible even during daylight. It fades from visibility almost instantly even at night. For a <1 sec duration shot the angle change will be <0.5 degrees - so no noticeable fan shape to the shot.
- Humans doing lab work with even modestly powered lasers (>0.005 W) wear eye protection that is optically opaque to the lasing
frequency to prevent loss of eyesight.
- Humans doing lab work with high powered lasers generally do so from bunkers/control rooms with no optical paths to the experiment.
"The Hard Kill" art from Atomic Rockets
Anyone able to see the target (whether looking or not) would be in extreme danger of permanent vision loss.
The reality is that the atmosphere absorbs and reflects every frequency to some degree. This is partially due to the fact that the atmosphere always contains fine particulate matter (aka aerosols). As energy is transferred from the beam to the atmosphere it heats the atmosphere to a plasma.
Due to the difference in emissivity between the plasma and surrounding atmosphere, the plasma acts as a waveguide and channels the beam. Since the plasma is a just super heated gas, that gas will move around due to normal gas dynamic forces such as wind and convection. Which means that even with perfect aim, targeting, and beam director stability, your beam will wander around and hit objects near the target.
The required beam dwell time depends upon how powerful your beam is. It's possible to incapacitate your target through lower powered, long dwell time heating shots. It's also possible for your beam to shatter or drill holes through targets using pulsed lasers. The sudden deposition of large amounts of energy in a very short period of time vaporizes a portion of the target's armor and creates a shockwave in the armor.
In general the long-dwell time lasers require less beam power than the high intensity shots. However, the high intensity shots may incapacitate with lower beam energy.
Effects of orbital motion
Even for the lower powered longer dwell time lasers, we're talking seconds to maybe 2 minutes of dwell time. In 2 minutes a laser satellite in LEO will move approximately 500 miles which equates to about a 55 degree change in incident beam angle (about 1 degree change / 2 seconds).
The US military attempted to shoot an orbiting satellite with it's Mid-Infrared Advanced Chemical Laser (MIRACL). This laser is a 1-2 MW Deuterium Fluoride laser. I believe an orbital laser to destroy a tank would need to be at a minimum this powerful and probably something like 10x or more powerful.
For very high intensity short duration shots, the beam would appear as a perfectly straight stroke of lightning and sound like one too. If the shot managed to explosively destroy a tank or bunker, you would get that detonation sound too.
Whether your beam is pulsed or continuous won't have much affect on its appearance. Either the beam will be powerful enough to cause the atmosphere to incandesce or it won't. Even if the beam is pulsed, the pulsing will be far too fast for the human eye to detect and the atmosphere will continue to incandesce (if it is hot enough to do so) between pulses anyway.
Laser test shoots down drone:
Note that you can't see the beam indicated that this is a "lower powered" beam incapable of shattering a tank.
What you don't know...
...about laser weapons can kill you. Nearly every laser technology has very low efficiencies. Most of the energy you pump into the laser gets turned into heat. Getting rid of waste heat in a laser weapon is going to be a big problem. In space, it's going to be a worse problem.
Note that laser cannon are notoriously inefficient. Free-electron
lasers have a theoretical maximum efficiency of 65%, while others are
lucky to get a third of that. This means if your beam power is 5,000
megawatts (five gigawatts), and your cannon has an efficiency of 20%,
the cannon is producing 25,000 megawatts, of which 5,000 is laser beam
and 20,000 is waste heat! Ken Burnside describes weapon lasers as
blast furnaces that produce coherent light as a byproduct.
From the Laser cannon section of the Atomic Rockets website (a must read for every world builder) - from a section on laser damage during a space battle.
A single pulse with a total energy of 100 MJ would have the effect of
the detonation of 25 kg of TNT. Everyone in the compartment who is not
shredded by the shrapnel will have their lungs pulverized by the
That same 100 MJ delivered as 1,000,000 pulses of 100 J each could
very well drill a hole. The crew see a dazzling flash and flying
sparks. Some may be blinded by the beam-flash. Anyone in the path of
the beam has a hole through them (and the shock from the drilling of
that personal hole could scatter the rest of them around the crew
compartment). Everyone else would still be alive and would now be
worrying about patching the hole.
Although it occurs to me that the jet of supersonic plasma escaping
from the hole being drilled could have the combined effect of a
blowtorch and grenade on anyone standing too close to the point of
incidence, even if they are not directly in the beam. The effect would
probably be similar to the arc flash you can get in high power, high
voltage electrical systems, where jets of superheated plasma can cause
severe burns from contact with the plasma, blast damage from the shock
waves, blindness from the intense light produced, and flash burns from
the radiated heat.
A continuous beam could have enough scattered and radiant heat to
cause flash burns to those near the point of incidence, along with
blinding those who are looking at the point of incidence when the beam
burns through. If it burns a wide hole, people die quickly when the
compartment explosively decompresses, throwing everyone into deep
space. If it burns a narrow hole, the survivors who can see can just
slap a patch over the hole to prevent the escape of their air.