The Inverse Square Law - Have I been lying all these years?

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The Inverse Square Law

Imagine you brought a gun into space, and fired a bullet.  Newton’s first law of motion says that bullet will travel forever in the same speed and direction, unless hindered by something like spacedust or diverted by the gravitational pull of a planet or star.  (You would also go flying backward when you fired the bullet, but that’s a subject for another time.)

Photons work the same way.  They keep going full strength until impeded.  That’s why the light from the sun isn’t attenuated by the time it hits the earth.

My parents, taken with a point-and-shoot
with pop-up flash.  Under these conditions, if the subjects are different distances away, only one will be exposed correctly.

And yet for years I’ve been teaching photography students about the Inverse Square Law, which states that the light gets weaker the further away it gets from the light source.  (And I gave several examples proving this law, like the image of my parents on the right!)  Have I been peddling a lie all of these years? 

The answer is both yes and no.  People experience the inverse square law all the time, especially back in the days when cameras had pop-up flashes and people used it as the sole source of light in a dark room.  (See example.)  Here, if there are multiple people in the shot, it’s important that all subjects are the same distance from the flash, for if the distances are different, only one of them will be illuminated “properly” – the rest will be too dark.