In article ,
Anthony E Anson writes:
|
| 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.

Really? I would have given it poor marks even at O-level!

Yes, the majority of the light hitting a dewdrop (or globe of
water) may well be reflected, but that is not the issue. The
issue is whether the PEAK intensity is enough to cause trouble,
and that will be dominated by the rays that hit near-normally.
It doesn't matter that we are talking about a disc 0.1 mm across,
as that is still much larger than a leaf cell.

I don't know the relevant formulae, so can't do the calculations,
but have observed light being concentrated by droplets. As you
should expect, the area behind the droplet is darker than that
which is fully exposed, but the very centre can be lighter.


Regards,
Nick Maclaren.