We are monitoring a dam for movements during the demolition of a nearby bridge. The monitoring points were set previous to our work and without our advice. Consequently, they are extremely difficult to observe and measure.
This story is about one particular monitoring point that is near the center of the spillway and is under water. The water is not that deep as it flows over the spillway. Perhaps 0.1 feet or less.
Anyway I am making observations with direct laser reflection, with prisms and with the level. I thought, why not try a direct laser reflection to the point under the water.
The results...
I got a reading that was the same as with the prism and an elevation that was the same as the level. In all it was a worthwhile experiment.
Historic boundaries and conservation efforts.
I'm surprised you found no difference.
The refractive index of water is around 1.34 with some variation in the last digit depending on wavelength of the light.?ÿ That is the ratio of speed of light in vacuum to speed in water.
That means an optical measurement straight through 0.1 ft of water should measure 0.134 ft, and at an angle (the bent stick illusion) would measure to a different point according to Snell's law..
I'm surprised too.
Perhaps the shallow depth, a steep angle of incidence and compensating errors are the explanation.
I am going to try to repeat the experiment tomorrow.
Historic boundaries and conservation efforts.
Then there is the fact that some actually do win the lottery ..
Update. We took a measurement with a ruler and?ÿ the depth of water over the spillway is only 0.02 feet.
Historic boundaries and conservation efforts.