Reflected Photons Challenge

(Taking Light Lightly)


On 12/26/2004, just as the Esprit Listserve was learning of the great Indonesian earthquake, Bryce Hand and Steve Kluge posted this mail on the list. Lost in the mass of communication regarding the ‘quake, we’ve reposted it here:


Colleagues -
Steve Kluge and I have been having fun off-list with a question prompted by all the discussion of the "moon illusion."
It all started several years ago when (staring at a crescent moon one night at the Jersey shore) I began wondering how much of the dim glow on the Earth-lit (³dark²) side might have come from the lighted kitchen window behind me.  How frequently would my eyes receive some of those reflected photons?  Would I see thousands per second?  One per year?
I entertained myself with the question many times after that, but never got around to playing with numbers...until a couple days ago, when my mentioning it to Steve got both of us thinking about it seriously.  Now, having taken independent approaches (and assuming a 60 W bulb), we've arrived at essentially the same answer.
Steve figured that some of you might like to tangle with it too (especially with the upcoming "Fermi Questions" competition at the Science Olympiad), and suggested that we pose the question on the listserv.  So here it is:

How much of the light reflected by the Moon comes from the 60 W bulb in the kitchen window behind you?  How frequently would your eyes receive some of those reflected photons?

(The empirical approach won't fly:  We won't trust your observation of how much the moon dimmed 3 seconds after you turned off the light!)
Enjoy!  And let us know what you come up with.

Bryce Hand and Steve Kluge

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