You remember the time as a kid when you set an ant on fire. You positioned your dad’s magnifying glass a few inches above the ground, adjusting the angle ever so slightly until the spotlight of refracted rays rested precisely on your target.* Then you waited.
It was innocent fun—except for some of us more sensitive folk—a sort of right-of-passage, backyard science experiment. But would you recall that lesson twenty years later while placing Fido’s clear glass water bowl on your deck?
Investigators of a house fire in Bellevue, Wash., last week are suggesting an elevated 11-inch wide glass bowl of water magnified the sun’s rays onto a wood deck, sparking a blaze that caused more than $200,000 worth of damage. Fortunately, nobody—including the two dogs—was injured.
To see if this dog bowl theory held water, Lt. Eric Keenan, the Bellevue Fire Department's community liaison officer, reconstructed the scene. He placed a partially-filled bowl on a wire stand nearly 14 inches above the sun deck at Bellevue City Hall. The atypical northwest spring conditions closely matched those on the day of the fire: a perfect “70 degrees and sunny, with light winds,” reported The Seattle Times. Sure enough, within about 15 seconds the small piece of cedar Keenan had set below the stand began to smoke under the sun’s concentrated rays.
Thomas G. Brown, a professor of optics at the University of Rochester, agrees that the scenario is plausible, at least under very specific conditions. The bowl must be transparent—preferably glass—with an overall convex shape, according to Brown. A wider bowl would need to be set further from the flammable material to concentrate the sun’s rays. (The resulting energy, however, would be far greater than that created through a small bowl – or maybe even your dad’s magnifying glass.) The skies must also be clear, dry and the sun shining from more-or-less directly overhead.
“Of course, the Seattle area is a little like Rochester—it is a rare and wonderful day indeed when we get that kind of direct sunlight out of clear skies,” Brown tells ScientificAmerican.com. “But if it is going to happen, late spring is the time.”
*Note (6/3/09): This sentence was changed after publication. As pointed out by one of our readers, the term "refracted" is more precise than "reflected" when referring to the sun's rays through a magnifying glass.
Picture by anjrued via Flickr