Originally posted by Ponderable
Nice work, sonhouse.
You probably won't habe a lake which is flat enough for this Kind of measurement. And you Need a quite strong laser for 20km water. Why not taking the air above the lake which is much more transparent.
If you take two People one adjusting the laer and one adjusting the detctor. Built the laser Level on the ground and look at what ...[text shortened]... should happen earlier than the height of the Intsllation due to the Point you described earlier.
I thought about that but there is a problem with that kind of measurement:
Atmospheric refractions can bend the laser beam from a perfectly straight path so that is a variable.
I put the laser underwater to eliminate atmosheric refractions. Of course underwater can do its own refractions so that would have to be taken into account but I think it might be less refractive than atmospheric refractions, if the temperature of the water is about the same at the surface and at 10 feet down.
From some sources it looks like the best bet would be a blue or ultraviolet laser. Maybe you would be limited to 2 km, if so the depth at 1 km would be about 10 inches.
So it all boils down to how far can a laser light be detected underwater. Note that does not have to be visible, only to be able to have a detector that can show the presence of the beam where the peak intensity is.
Dr. Jones from the US Naval research lab is doing that kind of work. Unfortunately they only get a beam to go 20 meters at this point in time, not sure what wavelength the laser is or the power, he says 'smaller than a desk' so they must be using a very high power laser, like those used for metal working, about 1 kw.
If so, my whole scheme falls apart. 20 meters would go off a straight path by only 1/10th of an inch, probably not measurable in the real world.
Oh well. Back to the drawing board.
If we could focus sound waves well enough it could be done with sonar but sonar as it stands causes a pretty much spherical wavefront, not very useful for this experiment.
It can be focussed though.
So maybe the on top of the water is more feasable if you can eliminate atmospheric refractions and diffractions.