Re: [VAL] Shiny Airstreams

[John Sellers <sellersj@xxxxxxxxxx>]

Gang,

Shiny Airstreams may have turned some grass brown and melted some
plastic, but I very much doubt that they have started any fires.

Your story about the beer bottle and Dr. J's story about the
water-filled jar sitting in the window both involve accidentally-created
lenses that concentrated the sunlight into a smaller area.  (In both
cases a cylindrical lens, which focusses the light onto a line, rather
than a point.)

You can also concentrate sunlight with a concave mirror, but the
polished external surfaces of an Airstream are either flat or convex.

Consider an Airstream with a vertical flat surface oriented toward the
sun.  Essentially the same thing as setting a large vertical mirror
facing in the direction of the sun.  Some point on the ground is going
to receive approximately twice the solar irradiance, half of it directly
from the sun and half of it from the sun's reflection.

Under the right conditions--really clear sky, no wind, no rain, twice
the usual solar irradiation can apparently kill the grass and melt some
plastics.  But it seems very unlikely that it could start a fire.

Best,
John & Barb Sellers
WBCCI/VAC #1587
1960 Pacer
Dayton, OH