> That's surprising but good to know!

I've seen the use of plain glass as an emergency signal mirror suggested a time or two.

{ Just to be clear <heh> - a glass signal mirror is definitely much brighter for the same area, as the 2nd half of this video demonstrates. The reflectivity of a glass mirror at normal incidence is about 11x brighter than a plain sheet of glass/plexiglass of the same area ( 90% reflectance vs. 2 surfaces x 4% per surface = 8% for the plain glass)). Since brightness falls off with distance squared (a bit faster due to atmospheric attenuation) the range is about 3.4x less for the plain glass. However, since the flash from a 2"x3" glass mirror is naked-eye detectable (admittedly, not very) at 24 miles naked eye, that leaves a lot to work with. Also, your scavenged sheet of glass may be larger than 2'x3". Finally, as quantified by the Fresnel equations the plain glass reflectance goes up at grazing angles, so the advantage of the mirror over plain glass is less at the (more problematic) high off-angles. }

Richard S. Hunter, the "father of the modern US military emergency signaling mirror"[1], said:

"Pieces of flat glass and flat plastic windshields and windows make good signaling mirrors." in:

Signaling With Mirror, July 1944, Air Sea Rescue Bulletin

and touches on this again in his 1946 article summarizing the development of US military emergency signaling mirrors in WWII:

Heliographic Signaling Mirrors, March 1946 Air Sea Rescue Bulletin

saying "Flashes of sunlight from ... or a plain piece of glass can be aimed quite easily and accurately by one on firm ground."

-R

[1] When requests came in for an improvement on the ESM/1 signal mirror that began issue in 1943, Hunter took the idea offered by Learned[2] and reduced it to practice in Feb 1944 (including sea trials), and went on to develop the "red bullseye" aimer that replaced it (and is used to this day by the French military), then the retroreflective mesh aimer featured in most US military emergency signaling mirrors since 1949.

Learned vs. Thompson

[2] Per the above, Learned first reduced his idea to practice in 1933, and from other stories, developed it while a Scoutmaster for his troop.