China: lasergun

[zombywoof]

The answer that I come up with is "it shouldn't need a windage adjustment, though a range adjustment could be helpful."

[raumvogel]

QUOTE (phoenix1 @ Jul 9 2018, 10:02 PM) The answer that I could look up is "it shouldn't need a windage adjustment, though a range adjustment could be helpful."

[ryujin]

QUOTE (SumVeritas @ Jul 9 2018, 06:06 PM) You're the physicist here

I'm no physicist, but I would imagine wind would affect a laser. Not enough to make a $#@!ing difference with a hand deployed laser gun, but any medium a light is traveling through would affect it. More so at sea level, and in high humidity.

[peet]

They probably have a huge number of sights lying around in China (largest army etc) so maybe they put one of these on top.

For green, red and blue lasers you can buy those laser safety googles. The guy in this YT movie claims his homebrew pewpew is 40 Watt.
https://www.youtube.com/watch?v=iVrJUbeuG44
There might be some discussion if 8 times a 5 watt laser is indeed 40 Watt.

[zombywoof]

QUOTE (Ryujin @ Jul 9 2018, 08:18 PM) I'm no physicist, but I would imagine wind would affect a laser. Not enough to make a $#@!ing difference with a hand deployed laser gun, but any medium a light is traveling through would affect it. More so at sea level, and in high humidity.

See, the thing is that light generally doesn't budge much regardless of the medium it's moving through. That's part of the conclusion we got from the Michelson-Morley experiments in 1887. Changing between one medium to another can be a little rough, but wind isn't going to change the homogeneity of air, if anything it would *remove* homogeneity, since it would act as a giant mixer $#@!ing up any layers that happened to form.

About the only conceivable problem I could imagine the packet of light going through are variable density pockets of air, but again wind serves to remove those variable density pockets, and also even if it didn't, the wind itself would create its own swirling eddies that would be impossible to predict, let alone correct for.

So even if the wind could affect the light, which I'm very confident it can't in most reasonable conditions (that is, normal atmospheric conditions rather than "superhero runs at .9 c right across your path the instant before you turn on the laser" type conditions), you wouldn't be able to compensate for it with "windage."

If you're still stuck on "light doesn't really care about its medium except insofar as what the medium is," consider that we identify windy days by random stuff being moved rather than by watching for shimmers or changes in the paths of light when air moves. Thermals provide some unusual visual effects, but that's largely due to the turbulent expansion of the heated air. This is caused, as I suggested before when I talked about how light doesn't care about its medium except insofar what the medium is, by refractive index gradients due to the fact that hot air is less dense than cold air. Unfortunately, again, wind would make these gradients unpredictable, if not so thoroughly mixing them as to render them absent.

[ryujin]

p1:

https://en.wikipedia.org/wiki/Mirage

[zombywoof]

The relevant sections:

"Light rays coming from a particular distant object all travel through nearly the same air layers and all are bent over about the same amount. Therefore, rays coming from the top of the object will arrive lower than those from the bottom. The image usually is upside down, enhancing the illusion that the sky image seen in the distance is really a water or oil puddle acting as a mirror.

Inferior images are not stable. Hot air rises, and cooler air (being more dense) descends, so the layers will mix, giving rise to turbulence. The image will be distorted accordingly. It may be vibrating; it may be vertically extended (towering) or horizontally extended (stooping). If there are several temperature layers, several mirages may mix, perhaps causing double images. In any case, mirages are usually not larger than about half a degree high (same apparent size as the sun and moon) and from objects only a few kilometers away."

Notice the movement of the air layers only exists in a "turbulent" manner. Turbulence is a set of disorder which, as I mentioned above, is not easily predictable under the best of conditions. Here also, is the section on heat haze:

"Convection causes the temperature of the air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. This produces a blurred shimmering effect, which affects the ability to resolve objects, the effect being increased when the image is magnified through a telescope or telephoto lens."

Notice again that the issue at stake is not the fact that the hot air is moving, but rather that the hot air has a different density than the surrounding air and thus a different refractive index.

The underlying principle is this: light's motion is only affected by the nature (density and constituent atoms of) its media, not the media's motion. Wind, therefore, can't "blow" a laser off course, and in any case is either consistent enough in your theater of operations to not need correcting for or is so inconsistent that correction is impossible at our current level of understanding regarding turbulent gas flow.

[zombywoof]

Also my first post suggests that wind will remove homogeneity. That is incorrect. I meant to say it would *increase* homogeneity. My only defense is that it was 4 am.

[cashto]

QUOTE (phoenix1 @ Jul 10 2018, 03:54 AM) See, the thing is that light generally doesn't budge much regardless of the medium it's moving through. That's part of the conclusion we got from the Michelson-Morley experiments in 1887.

No, that's not what the Michelson-Morley experiment demonstrated. Light quite obviously changes direction (refracts) in response to different media. Shine a laser through a glass of water until you're satisfied that this is the case.

What the M-M experiment demonstrated is that there is no fixed absolute frame of reference that light travels in, no lumiferous aether which 'carries' light. If there was, then light would get a boost from earth's own movement through this aether in one direction, and a corresponding retarding effect when moving in the other direction -- similar to an airplane flight in a constant wind, it will always make a round-trip journey in longer time than if there is no wind (you will always lose more time flying against the wind on the return trip than you will gain flying with the wind).

As far as the motion of the material affects the path of light, I don't know (does shining a light through moving water (in laminar flow) have a different result than shining a light through still water? If it does, the effect would be minimal (a billion km per hour in one direction, and at a right angle, say, 30 km per hour ...)

Anyways, refractive effects would dominate (such as in a mirage) -- followed by relativistic effects of gravity.

But to answer your question -- no, there is no need to make an adjustment for any of these affects, since the image formed by the light coming to your eyeball, and the light being fired from the laser gun, are going to be affected by all these effects in exactly the same way.

[zombywoof]

QUOTE (cashto @ Jul 10 2018, 11:33 AM) No, that's not what the Michelson-Morley experiment demonstrated. Light quite obviously changes direction (refracts) in response to different media. Shine a laser through a glass of water until you're satisfied that this is the case.

What the M-M experiment demonstrated is that there is no fixed absolute frame of reference that light travels in, no lumiferous aether which 'carries' light. If there was, then light would get a boost from earth's own movement through this aether in one direction, and a corresponding retarding effect when moving in the other direction -- similar to an airplane flight in a constant wind, it will always make a round-trip journey in longer time than if there is no wind (you will always lose more time flying against the wind on the return trip than you will gain flying with the wind).

The M-M experiment demonstrated that ether, if it exists, does not pass by the earth at a velocity very different than the velocity of earth's passage through a vacuum.

Direct quote from Michelson: "The Experiments on the relative motion of the earth and ether have been completed and the result decidedly negative. The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe – the maximum displacement was 0.02 and the average much less than 0.01 – and then not in the right place. As displacement is proportional to squares of the relative velocities it follows that if the ether does slip past the relative velocity is less than one sixth of the earth’s velocity."

Ether was later rejected because its properties ended up being far too fantastic and failed to account for experimental findings. It was replaced by special relativity which described the speed of light through a consistent medium being constant, and necessarily slower than the speed of light through a vacuum, which is, according to special relativity, not possible.

Your "but what about light through a glass of water" misses four very important things:

1- the boundary condition between the air and the glass
2- the boundary condition between the glass and the water
3- the boundary condition between the water and the glass
4- the boundary condition between the glass and the air

For optics, only the boundary conditions matter. Thus, the media itself doesn't really matter... it only matters when you reach a boundary condition.


QUOTE As far as the motion of the material affects the path of light, I don't know (does shining a light through moving water (in laminar flow) have a different result than shining a light through still water? If it does, the effect would be minimal (a billion km per hour in one direction, and at a right angle, say, 30 km per hour ...)[/quote]
It does not have a consistent effect. Stare at the sun on a windy day. Or don't do that, but feel free to use some kind of lens to focus the light from the sun on a windy day and watch how the focused spot doesn't move or waver with gusts of wind. It's possible, because of the way turbulent flow works, that local pockets of variable density air can make for minor deviations, but the random nature of those local pockets would render the outcome virtually impossible to predict and would likely, statistically speaking at least, have no net effect.