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From: Trevor J. Kenchington (no email)
Date: Fri Jul 16 2004 - 17:19:11 EDT
Fred Hebard wrote:
> I am specifically wondering whether a ship could ever appear to sink
> below the horizon even though it actually was above it. My
> understanding is that normally ships appear higher than they really
> are.
Fred,
I think you are missing one of the fundamentals of optics.
The image of a ship that is created by a mirage, like the image of
your face when you look in a mirror, does not exist at the point where
you think you see it. Technically, it is a "virtual image". [Other
optical systems do create images where we think we see them -- a slide
projector, for example, does create an image, which we can see if we
place a screen at the right place (or else twiddle the focus knob on the
projector to move the image to where we had already placed the screen).]
Since the image is a virtual one, it cannot sink _behind_ the horizon
because it doesn't actually exist anywhere near the horizon. What is
happening is that light rays heading away from the ship are bent (by
refraction) and eventually reach your eye while heading upwards
(assuming cold air over hot water -- or more realistically hot land).
Your eye/brain system then interprets those rising rays as coming from
an object below the horizon but nearer to you than the horizon.
And that is the conventional type of mirage, familiar in hot deserts.
[We usually think of such a mirage in terms of seeing an image of the
sky which resembles a lake. However, while driving down the highway here
in Nova Scotia a few days ago, I saw clear images of on-coming vehicles
and even the yellow line down the centre of the road on the next upward
slope. (Nova Scotia gets strong sunshine and cold air, giving us
well-developed mirages quite often.)]
You earlier wrote:
> I am trying to get at the question of objects located on earth, or
> near, rather than stars. Both of your examples, low and high latitude,
> appear to involve the image being located above its source. I am
> wonder whether there are any examples of the source of an image being
> on top. (In the case of light reflected off ice, I would consider the
> point of reflection to be the source). But I may have a very poor
> understanding of desert mirages.
The above anecdote should answer the point about objects on Earth. The
low-latitude, cold-air-over-hot-land type of mirage (the commonest type)
does not place the image above the object but below it, as with sky
being seen apparently on the ground. Aside from situations where you
might see the Sun reflected as in a mirror, light reflection from ice
just makes the ice look bright white. That doesn't involve refraction or
a mirage. However, in Arctic conditions, the reflected light which heads
obliquely upwards can then be refracted such that it comes back down
again. The observer whose eyes receive those downward-tending rays will
perceive the brightness of the ice as being in the sky, where the light
rays appear to be coming from. That, as best as I understand it, is ice
blink.
Finally: The only way for a mirage to cause the horizon to obscure a
ship, if that is what you are asking, would be for a
warm-air-over-cold-ocean type of mirage to raise an image of the ocean
surface into the sky, confusing the outline of a ship (likely one
hull-down over your horizon). I haven't heard reports of that happening
but I don't see why it wouldn't under some conditions.
Trevor Kenchington
--
Trevor J. Kenchington PhD
Gadus Associates, Office(902) 889-9250
R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251
Nova Scotia B0J 2L0, CANADA Home (902) 889-3555
Science Serving the Fisheries
http://home.istar.ca/~gadus
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