In article ,
Anthony E Anson writes:
| The message
| from Janet Baraclough contains these words:
|
| I wash my African violets under the tap (tepid water not cold) to
| rinse dust off their hairy leaves, and they don't seem to mind at all.
| You can probably get rid of the greenfly after a few washes. Just let
| the plants dry off out of the sun so the water drops don't make scorch
| marks.
|
| 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.
|
| 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.

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.

All good O-level physics.


Regards,
Nick Maclaren.