Hallmark recalls fire-starting jumbo snowman snow globe

December 25, 2008 by Fred Bortz

Hallmark has announced a recall of its jumbo snowman snow globes due to the possibility of fire.

If only the designer had read the same newspaper article as a young lady who was working on a report for the laser teaching center at SUNY Stony Brook.

Hallmark should have hired Kira Schultheis, the young lady who did the linked report on "The Many Uses of Crystal Balls", including serving as ignition sources for house fires.

Ms. Schultheis was inspired to do her project by an article she read, probably in a newspaper.

For those who are interested in the science, here's my summary:
A sphere does not focus light as sharply as a properly ground lens, but it can concentrate it well enough to cause a problem. According to Kira's formulas, which look sensible to me, a water-filled sphere (estimated index of refraction 1.4), has a "back focal length" of approximately 0.75 times the radius, so for the 11 cm. diameter sphere she was using, that is about 4.1 cm. or 1.6 inches. A piece of combustible tissue wrap at that distance from the globe could easily ignite.

The problem is much less severe with a smaller globe for two reasons. The more significant reason is that the light-collection area is smaller by the square of the ratio of the radii, so a sphere half the size would focus only 1/4 as much energy. Secondarily, the focal length is proportional to the radius, so the danger point is closer to the globe, where it is less likely that another object will be placed.

So the lesson for this Christmas is never trust a fortune-teller, and never buy a jumbo snow globe when a smaller one would do.

Fred Bortz, author of science books for young readers