Re: How far can we see reflected light?

The absolute threshold at which we can detect light in complete darkness 
is indeed amazingly small. Galanter 1962 measured it at 10 quanta of light 
energy - equivalent to being able to detect a candle flame  at a distance 
of 30 miles on a dark clear night. So it is not surprising that you can 
see the reflected beams of powerful car headlights.

He also measured the absolute threshold for touch as being equivalent to 
the wing of a bee falling on your cheek from a distance of 1cm! 
However,our ability to sense is very heavily dependent on what is going on 
around us as our brain adapts to the current visual/noise etc conditions. 
More recent research has been toward understanding the brain's role in 
actively directing perceptual attention rather than in measuring absolute 
thresholds which has not found to be too helpful as these conditions never 
occur in real life.

Ref Galanter E. (1962) "Contemporary Psychophysics" in New Directions in 
Psychology edited by Brown, R., Galanter, E., Hess, E.H. and Mandler,G. 
Published by Holt, Rinehart and Winston, NY.