12-05-2003, 12:45 PM
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greenfly
The message
from (Nick Maclaren) contains these words:
Scorch marks due to droplets of water is an old - er - partner's tale,
| Janet. You cannot concentrate the sun's rays (either direct or diffused)
| with a sphere or an hemisphere.
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| I remember Geoffrey Smith pointing that out on GQT ages ago, and
| thinking about it, he was quite correct.
Eh? Oh, yes, you can! If you look at suitable museums, you will
see that globes of water were used to concentrate rushlight etc.
so that seamstresses could work in the winter.
But not tangential to the surface.
A sphere of something with a higher refractive index than air is
a condensing lens, and water has an index of c. 4/3. Because the
focal point is not ON the sphere, actually scorching can occur
only if the droplet is held away from the surface (say, by hairs)
or the sunlight is slanting to the surface.
I think you'll find that the focal point of the light emanating from a
sphere of water is immaterial - the angle of incidence of the light
hitting the water/air interface is such that most of it is reflected,
not refracted, which is why dew sparkles...
All good O-level physics.
Good A-level physics too.
--
Tony
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