12-05-2003, 08:32 PM
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greenfly
The message
from (Nick Maclaren) contains these words:
| 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.
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.
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.
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.
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.
--
Tony
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