A transducer is a device or component that converts signals and, in conversation, that almost always means analog mechanical<->electrical. They’re quite easy to visualize (heh) in audio: the amplifier’s tightly controlled voltage is converted to mechanical bumps & wiggles in the air by a transducer we call a speaker. The reciprocal also works: a sound directed at a tranducer shakes its surface and induces, transducing an electrical response that is measured & logged: a microphone. The front seats of most cars have weight sensors, load cells: transducers, for air bag occupant detection purposes. A speedometer might be a transducer, lots of engine sensors, etc.
A radar also does some transducing at the tx/rx site(s) but we don’t usually use the term for radio or light where emitter or receiver or just ‘sensor’ is often used.
You seem to have the notion of total internal reflection backwards. You get total internal reflection only hen going from a medium with high refractive index to one of low refractive index, such as going from water (n = 1.333) to air ( n = 1.0003). You don’t get the effect going from air into water.
You are correct. I’ve been spending too much time inside the waveguide, need to get outside more.
There is no critical angle in this case. However, Snell’s Law still applies to determine the angle of refraction, which along with Fresnel equations determine refracted (into the water) vs. reflected power. For light normal to the water (straight down) the reflectance is:
R = ((n1 - n2)/(n1 + n2))^2
where n1 is the refractive index of the air (1.0) and n2 is that of the water (~1.33) leading to a reflectance of 2% - very little is reflected from the surface of the water, which is why the rangefinder doesn’t work. Although is the water is clear enough (and the water is shallow enough) you might be able to measure the distance to the bottom (with an error caused by the refractive index of water when the rangefinder assumes air).
That’s a far worse reflectance than I had naively assumed would be the case. But it does sorta explain some more data I gathered since my last post. To wit:
I went to the other taller bridge which also had slightly clearer but much rougher water. For round numbers from the clearance guides it’s 35-40 feet = 12-13 yards to the water surface. Plus my eye height, so ~15 yards is a reasonable rangefinder reading. And is well above the device’s min range.
Shooting more or less straight down normal to the plane of the bulk water surface and in the shade I usually got no reading. But about 1 in 10 would return a reasonable value. But with a ±1ish yard noise component. So ~>10% variation from reading to reading. Meanwhile the actual amplitude of the predominant surface waves from wind and current was ~3", or up to ~10" in a boat wake. So I’m interpreting that as just barely enough signal coming back to process, and maybe some vagaries about how it might be corner-reflectoring between adjacent waves.
When the water was stirred up with white foaming wake from a passing boat I got a much better response than from the plain green/brown undisturbed water near a wake. The noise component from reading to reading also seemed much reduced. Shooting at a wake almost always gave a reliable reading.
If our OP is looking to know their height above water from one deck or another of a cruise ship underway, I predict the continuous white wake directly alongside the hull should give an adequate return. I also predict that the smooth water alongside while tied up to the dock, especially on the lee side of the ship, will have no useful return.