How many visible photons per second does a light bulb emit? (Thanks to Arav Singh, a REYES student from Arlington, VA, for suggesting the question.)

How much does the U.S. spend on residential light bulbs (both the bulbs and the electricity) every year? (Thank to Hollis Williams of the University of Warwick, UK, for suggesting the question.)

Look for the answers online at under “Browse,” at the very end of the current issue.

Question suggestions are always welcome!

For more Fermi questions and answers, see Guesstimation 2.0: Solving Today’s Problems on the Back of a Napkin, by Lawrence Weinstein (Princeton University Press, 2012).

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