In article ,
Anthony E Anson wrote:

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.

It is some 45 years since I did A-level physics, but my long-term memory
is pretty good, despite my lamentable performance remembering people's
names, birthdays etc. I drew the diagram (twice, once as a check) and
the focus for the light which is admitted to the sphere is approximately
the radius of the diameter of the sphere, measured from the centre.

It is only 40 years since I did mine. I don't understand what you
mean, but I think that it is about a radius distance from the surface
of the sphere. Whatever. It doesn't matter, as long as it exists.

However, only a small proportion of the light which falls on the surface
passes through - much of it is reflected. Some of what does enter is
absorbed, some is reflected within because the angle of reflection of
light passing from water into air is such that much less light than
enters can directly exit.

As I said, that isn't the point. What matters is the proportion of
the NEAR-NORMAL light that is transmitted, and that is going to be
above 50%, perhaps 80%. It drops off to zero at the periphery, but
that is irrelevant.

Then, in the very unlikely event of any part of the leaf touching the
focus, your whole hypothesis falls over because the sun continues to
move the goalposts.

Which is why the sun focussed through discarded bottles never causes
fires, I suppose. You do know that it does, don't you?

The point is that (say) all of the radiation passing through a circle
of radius 0.1 mm is concentrated into a circle of (say) 0.01 mm,
multiplied by the transmittance (say 0.5). This is 50 times as strong
as the incident sun, and is quite capable of doing cell damage in
seconds.

At that distance (say a 1 mm radius droplet), the rotation of the
earth means that the focus will move 0.01 mm in 45 seconds, so it
will burn a path through the cells.

And, just to complete the argument, all of the rays that I am
considering hit the droplet within 3 degrees of normal, and so the
reflection is definitely small and the focussing is good.

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.

Angle of refraction = angle of incidence x refractive index. For my
diagram I've used your figure of 3/4 - 4/3 for refractive index, which
seems a little high to me. However, just look at it from the commonsense
angle - if it were possible to damage a plant's surface in this way
there would be evidence of it occurring in RL - er - Real Life - and
there isn't.

Sigh. Not THOSE formulae, which are elementary, but the proportion
of light transmitted and reflected at various angles.

The refractive index is 4/3 at c. 650 nM - i.e. red light.

I haven't seen the damage in real life, but that is largely because
the conditions for it to occur are rare in the UK. I do believe
that it happens, though I agree that it isn't the major danger that
many books make it out to be.


Regards,
Nick Maclaren.