On 5 Dec 2003 16:38:12 GMT, Robert John
wrote:
.... text removed...
Not ever having seen one, I can't quite get my mind
around how a hole in the centre helps you aim it.
If the sun is 45 degrees left and you aim at the target
aircraft, won't the reflected beam be 45 degrees right
of the target?
I'm not arguing - I'm sure they work, I'd just like
a simple explanation of how!

This may not be simple, but most of the real signal mirrors I am
familar with have a hole in the backside that is surrounded with a
circle of retroreflective material (like 3M Scotchlite), typically red
in color. The sunlight is reflected off the mirror surface normally.
Where the sunlight hits the Scotchlite, it is retroreflected directly
only the incoming path. A small portion (about 4%) of this reflected
sunlight, now colored red by the Scotchlite, is in turn reflected
backwards again by the front surface of the mirror (toward the user
looking through the backside of the mirror). The usual optics rules
for mirrors (angle of incidence equals angle of reflection) means that
this retroreflected (red) radiation appears to come along the exact
angle as the outgoing reflected sun light. The bottom line is that
when you look outward through the small hole in the backside of the
mirror, all you have to do is put the small red spot you see on the
target you are trying to signal. The Scotchlite properties and the
rules of optics reflection assure that the red spot and the reflected
sunlight are pointing in the exact same direction.

Sorry UseNet does not permit embedded graphics.

Bob