On Sun, 2 May 1999, Jim Morrison wrote:
> I have a desk "Space Calendar" that has a tear-off page with an outer space
> related picture for each date. The picture for April 30, shows the Space
> Shuttle on the ground at Edwards Air Force Base with a perfect reflection of
> the vehicle in what appears to be a lake. It is not a lake, but a mirage
> caused by the extreme heating of the of the ground.
>
> Can anyone explain what is happening that causes such a perfectly reflected
> image?
> James E. Morrison
> Astrolabe web pages at: http://myhouse.com/mc/planet/astrodir/astrolab.htm
Hi, Jim!
I'm not perfectly clear here, whether there is something really unusual
about this particular mirage, or if you were asking about mirages in
general. Maybe we could come up with some way to incorporate a mirage as
the image-forming element in a really *big* sundial! :{)
Mirages are commonly formed where there the sun heats a road or desert
surface, creating a super-heated layer of air in contact with the road.
Overlying the hot, still, layer is a larger body of relatively cool air.
The hot layer is less dense (which, by the way, makes the whole thing a
bit unstable), and therefore has a lower refractive index than the cooler
air above it. Grazing light rays from a distant source (the Shuttle) that
would just barely enter the hot layer are refracted back up, in a total
reflection, similar to that on the underside of water surface, or inside
an optic fiber. To the eye/brain system, the only natural thing we "see"
create a reflection in that position is a body of water. Add the slight
rippling from the hot layer interfacing with breezes, and you see why
desert travellers were led astray, following that elusive image of water,
right into the hottest, flat, desert basins!
Dave
