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On May 6, 2021, at 12:18 PM, John Denker via Phys-l <phys-l@mail.phys-l.org> wrote:
On 5/6/21 9:26 AM, Paul Nord wrote:
Oftentimes at a point of contradiction like this there
is a deeper understanding to be gleaned. Help me out there. To what new
insights might this lead?
https://www.youtube.com/watch?v=JsoOlPpDv9I [corrected]
Just because it says it's a "rangefinder" doesn't mean it
measures distance. It doesn't. It can't. It's obviously a
time-of-flight measurement ... from which distance could
be inferred *if* you knew the index of the medium.
The rangefinder assumes the medium is air. It will be off
by a small but noticeable amount on a hot day, because the
index changes when the density changes. You can get some
not-so-cheap rangefinders that measure time-of-flight at
two different frequencies, so they can estimate of the
density, so they can make an accurate correction.
And if the medium isn't air at all, then all bets are off.
Since the rangefinder is surreptitiously multiplying by
c/n (where n is about 1.0003) you can de-mystify the
situation by dividing that back out and reporting the
time-of-flight, which is what's really being measured.
The contradiction arises when comparing an estimate
based on triangulation (which could measure distance,
but does not, because of refraction at the interface)
versus a measurement based on time-of-flight )which is
only pretending to be distance). Both measurements are
wrong ... in opposite directions.
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