Half a croissant, on a plate, with a sign in front of it saying '50c'
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Laser rule

Because lasers RULE
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If you switch a laser on very briefly, you can produce a pulse of light that is only 1cm long. If you switch the laser on and off at this very high frequency you can produce a stream of pulses each 1cm long and seperated by 1cm gaps.

This is not very useful as a measuring instrument because it is travelling at c

Howver two identical devices aimed at each other coaxially will produce a static pattern of light and dark zones which can be used as a handy measuring device.

pocmloc, Apr 28 2017

Light My Inches Light_20My_20Inches
For those who don't follow the metric system [theircompetitor, Apr 28 2017]

[link]






       I'm not following the physics here. Why will they produce a static pattern?   

       Also, to work, the light would have to fall on something - you can't see light, as such.
MaxwellBuchanan, Apr 28 2017
  

       It seems to me that interference would work better.
nineteenthly, Apr 29 2017
  

       Since light speed varies with the refractive index, and this in turn varies with air pressure, RH, and temperature expect a 50-200ppm or thereabouts error, depending on conditions, using light.
Ling, Apr 30 2017
  

       That means it might double as a means of checking characteristics of the substance through which it's transmitted. Or, you could just have a transparent ruler between the devices, and maybe get rid of the devices.
nineteenthly, Apr 30 2017
  

       Imagine two identical long goods trains passing each other at the same speed but opposite velocities. Observers standing at the trackside can see through the gaps in the nearer train, only at the moment it passes. but their view through will be blocked at that moment by a wagon of the far train. Except for some viewers, who are opposite the places where the gaps momentarily line up. I submit that those particular observers will see through the gap in the far train every time a gap in the near train is level with them, whilst every other observer will never see through both trains at once.
pocmloc, Apr 30 2017
  

       Green lasers are visible in air due to Rayleigh scattering
pocmloc, Apr 30 2017
  

       WOW!!!   

       ....+....
....+....
+++++++
(etc. avoiding the drawing of a cross...)
pashute, Apr 30 2017
  

       Of course it's fundamentally flawed due to basic stupidity on the part of the author.
pocmloc, Apr 30 2017
  
      
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