The intensity of light changes with distance as an inverse-square law.


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Theory:

The intensity of light drops off as a function of distance from the source as an inverse-square law.  To whit:

I = Io/d2

where I is the intensity, Io is the intensity at the source, and d is th distance from the source to the observer.  This comes straight from Maxwell's equations for electromagnetism.  


Apparatus:

  • PocketLab Voyager
  • portable smart device (phone or tablet)
  • light source

Procedure:

  • Set up your light source.  Ideally, it should be the dominant light source in the room.  Turn off as much outside light as possible.
  • Setup the PocketLab Voyager to simultaneously measure distance (rangefinder) and light intensity.
  • While taking data, physically move the PLVoyager towards and away from the source.  The light detector and the rangefinder both measure from the orange front face of the enclosure.
  • Once you have repeated the motion several times, stop the data taking and save the data.
  • Plot the light intensity versus distance and see the 1/d2 dependence,

 

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