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The idea with a plain mirror (and a sunny day) is that you've got to hold the fingers of your other hand in front of the mirror, with a gap between the fingers, so you can see the reflected beam of sunlight on your hand/finger gap and aim it toward your target (usually an aircraft).
The reflected flash of sunlight is strong enough to be noticed by the aircraft... may not be as noticeable from a boat on a glistening sea, that would have to be real close.
Obviously needs a) a sunny day; b) an aircraft that's paying attention, and c) previous practice on your part.
The gadget that wxyz is linking to promises to help with the aiming part. Don't see a price, though...
No, it really works. You can aim and hit a traffic sign at a mile or so and see it light up.
How is different from taking a CD and looking through the hole at the object you want to target??? You would be able to see the target but the mirror ankle could deflect the sun in all different directions.
Haven't you ever sat in a meeting near the window only to "accidently" have the sun bounce off the crystal face of your watch and shine in your friends eyes, while you casually act like your paying attention?
NOTE: We are often asked about, or see the recommendation online and elsewhere, the use of a CD-ROM as signal mirror. It is shinny, reflects light, has a hole in the center, and thus looks somewhat like a signal mirror. Moreover, many of us have lots of useless CDs around, AOL continues to send many of more of them, thus its appeal.
In tests a CD proved to be only about 20%-25% as effective (distance and brightness at distance, judged subjectively) as a 3 x 5 mil-spec plastic signal mirror, a bit more effective, but not even 50% compared to a small 2 x 3 mil-spec plastic signal mirror. It would compare worse against higher quality mirrors.
From an operational persepctive, in an After-Action Report of a major SAREX (Search and Rescue Training Exercise) conducted in 2001 by the Colorado Wing of the Civil Air Patrol, the conclusions were, "that "victims" who had never used a signal mirror (with the aiming hole in the middle) were able to use them effectively, while CD's (AOL etc.) are useless as signal mirrors."
It also has the disadvantage of requiring two hands to aim it as compared to a signal mirror with an integrated aiming device, the hole is not an aiming device. The mil-spec plastic mirrors also float, the CD may or may not, and certainly not as well. Given that a good signal mirror is not very expensive and is the most effective daytime (sunlight) signaling device, using a CD hardly seems worth the savings.
Not to say you couldn't be rescued using a CD. People have been rescued using the hologram on a credit card to signal with and the CD is far superior to that small reflective surface. A CD makes a decent improvised signal mirror compared to many other options and since they are essentially free, it may be worth carrying some for other members of your crew or party to use, multiplying your signaling capability. However, we cannot recommend a CD as a primary survival signaling device