The message
from Anthony E Anson contains these words:

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.

Sometimes real life contradicts physics theory :-).I have a
collection of those clear solid glass spheres with a few air bubbles
inside the glass. When we moved here I set them on a sunny south
windowsill, until I found that when the sun shines through the window
and through the glass balls, there's a focussed hotspot; hot enough that
you wouldn't want to keep your hand there for long.

Janet

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.

The message
from Janet Baraclough contains these words:

Sometimes real life contradicts physics theory :-).I have a
collection of those clear solid glass spheres with a few air bubbles
inside the glass. When we moved here I set them on a sunny south
windowsill, until I found that when the sun shines through the window
and through the glass balls, there's a focussed hotspot; hot enough that
you wouldn't want to keep your hand there for long.

But the refractive index of glass is much higher than that of water.

--
Tony
Replace solidi with dots to reply: tony/anson snailything zetnet/co/uk

http://www.users.zetnet.co.uk/hi-fi

The message
from (Nick Maclaren) contains these words:

\snip\
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.

But it isn't - I'd be surprised if 15% of incident light exited
directly. Without a lot of calculations and data which I haven't got to
hand - and may not have at all now, I couldn't work out a figure.

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 refractive index of glass and water are entirely different, as are
the shapes of (say) the broken bottom of a bottle and a globe.

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.

But your arithmetic is wrong: we are talking about spheres, not circles.
If the light incident on a globe of radius ·1mm is concentrated in to a
circle of radius ·01mm you have to take the amount of light which passes
through, which is way less than 50%. The greatest loss is caused by the
inability of light to escape from water to air if the angle of incidence
is over a certain figure, and this is a lot less than yhe other way
round.

Have you ever looked at the surface of a swimming pool from under water?

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 it doesn't. Look at any leaf you like after the sun has been sining
following a light shower.

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.

They can't. It is in the nature of droplets to have a surface which is
curved in two planes.

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.

another sigh
I would have thought that the conditions in the British Isles would have
been ideal for the trials.
/sigh

--
Tony
Replace solidi with dots to reply: tony/anson snailything zetnet/co/uk

http://www.users.zetnet.co.uk/hi-fi

In article ,
Anthony E Anson wrote:
The message
from (Nick Maclaren) contains these words:

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.

But it isn't - I'd be surprised if 15% of incident light exited
directly. Without a lot of calculations and data which I haven't got to
hand - and may not have at all now, I couldn't work out a figure.

I would be EXTREMELY surprised if the proportion of near-normal light
that is transmitted is less than 50%.

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 refractive index of glass and water are entirely different, as are
the shapes of (say) the broken bottom of a bottle and a globe.

Eh? 1.5 versus 1.33. Not that different.

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.

But your arithmetic is wrong: we are talking about spheres, not circles.
If the light incident on a globe of radius ·1mm is concentrated in to a
circle of radius ·01mm you have to take the amount of light which passes
through, which is way less than 50%. The greatest loss is caused by the
inability of light to escape from water to air if the angle of incidence
is over a certain figure, and this is a lot less than yhe other way
round.

Sigh. I am talking about a sphere of (say) 1 mm diameter. The light
that passes within 6 degrees of normality will enter within a circle
of 0.1 mm diameter. Elementary geometry. I do NOT believe your
claim that most such near-normal light is reflected - if it were,
you couldn't look down at the bottom of a shallow pool with the sun
overhead.

Have you ever looked at the surface of a swimming pool from under water?

Yes.

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 it doesn't. Look at any leaf you like after the sun has been sining
following a light shower.

The chances of the focus being close to the leaf are low. As I said,
I have seen the focussing effect with water droplets, though I have
not seen it happen precisely enough to cause tissue damage. That
does not mean that it doesn't happen.

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.

They can't. It is in the nature of droplets to have a surface which is
curved in two planes.

Ye gods and little fishes!

If you shine a parallel beam at a sphere, the rays that hit within
N degrees of normal define a circle on the surface of the sphere. All
of lens theory is based around the theory of near-normal rays - well,
at A-level, it is - it gets a bit more complex later on.

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.

another sigh
I would have thought that the conditions in the British Isles would have
been ideal for the trials.
/sigh

Clearly. But that is because you haven't looked deeply enough into
the issue. It is extremely rare that the sort of showers that form
many droplets on leaves are followed by strong sun - it is far more
common for the resulting sun to be weak or even watery. This is
not true in the tropics.

Remember that even direct sun at midsummer in the UK is rarely more
than 50% of the earth's insolation, and it is common for it to drop
to 10% or less even on bright days. Yes, we really DO get that
little sunlight here, largely because of the amount and wetness of
the atmosphere that the light has to travel through.


Regards,
Nick Maclaren.

The message
from (Nick Maclaren) contains these words:

\snip\
Sigh. I am talking about a sphere of (say) 1 mm diameter. The light
that passes within 6 degrees of normality will enter within a circle
of 0.1 mm diameter. Elementary geometry. I do NOT believe your
claim that most such near-normal light is reflected - if it were,
you couldn't look down at the bottom of a shallow pool with the sun
overhead.

Please keep up at the back there. Light incident on a flat surface will
all enter up to a much wider angle than light exiting.

A globe has not got a flat surface, and the angle of incidence of light
falling on that surface is modified rapidly the futher from the axis of
incidence on the sphere.

But the main filter is when the light which *DOES* get through meets the
(effectively) concave mirror of the bottom of the droplet.

Have you ever looked at the surface of a swimming pool from under water?

Yes.

And what did you see beyond it? If you remember seeing very much it
would be the triumph of imagination over memory.

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 it doesn't. Look at any leaf you like after the sun has been sining
following a light shower.

The chances of the focus being close to the leaf are low. As I said,
I have seen the focussing effect with water droplets, though I have
not seen it happen precisely enough to cause tissue damage. That
does not mean that it doesn't happen.

Well, I hold that it does. As do the experts on Gardeners' Question Time.

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.

They can't. It is in the nature of droplets to have a surface which is
curved in two planes.

Ye gods and little fishes!

If you shine a parallel beam at a sphere, the rays that hit within
N degrees of normal define a circle on the surface of the sphere. All
of lens theory is based around the theory of near-normal rays - well,
at A-level, it is - it gets a bit more complex later on.

Ye Gods and bigger fishes! Any parallel light striking a sphere will
illuminate totally an area of a median cross-section of that sphere.
What is this red-herring of 'N degrees?

However, the actual curvature of the surface drops away as a function of
pi and the diameter, rapidly increasing the angle of incidence of light
falling on it.

But I can't see our differences being resolved without the application
of Very Hard Sums.

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.

another sigh
I would have thought that the conditions in the British Isles would have
been ideal for the trials.
/sigh

Clearly. But that is because you haven't looked deeply enough into
the issue. It is extremely rare that the sort of showers that form
many droplets on leaves are followed by strong sun - it is far more
common for the resulting sun to be weak or even watery. This is
not true in the tropics.

Remember that even direct sun at midsummer in the UK is rarely more
than 50% of the earth's insolation, and it is common for it to drop
to 10% or less even on bright days. Yes, we really DO get that
little sunlight here, largely because of the amount and wetness of
the atmosphere that the light has to travel through.

Ah, but perhaps I have the advantage here, having lived in the North
West of Scotland, and where my occupatin has taken me to altitudes of
over 3,000 feet, where minute droplets of water are rather more common
than is desirable for personal comfort.

BTW, if any battle fails to be joined on what you may hope is a
contentious issue, don't think that I've parked my armour under the bed
and stabled the destrier: our ISP has recently 'improved' the newsfeed
and a few posts are evaporating in the midday sun.

Either I may not see your nonsense or you might not see my wonderfully
reasoned arguments ;-p

(Ask Janet & John, Helen Vecht or Anne Jackson)

--
Tony
Replace solidi with dots to reply: tony/anson snailything zetnet/co/uk

http://www.users.zetnet.co.uk/hi-fi

In article ,
Anthony E Anson writes:
|
| But I can't see our differences being resolved without the application
| of Very Hard Sums.

They are actually Very Easy Sums - at least for a mathematician.
So easy that they cound as mental arithmetic exercises, in fact.

| Remember that even direct sun at midsummer in the UK is rarely more
| than 50% of the earth's insolation, and it is common for it to drop
| to 10% or less even on bright days. Yes, we really DO get that
| little sunlight here, largely because of the amount and wetness of
| the atmosphere that the light has to travel through.
|
| Ah, but perhaps I have the advantage here, having lived in the North
| West of Scotland, and where my occupatin has taken me to altitudes of
| over 3,000 feet, where minute droplets of water are rather more common
| than is desirable for personal comfort.

The water does seem to have got places that it shouldn't. Yes,
this country has plenty of water droplets, though not all that often
the sort that settle individually on leaves that are not moving.
Even a very light wind will prevent the focus remaining in one
place.

But where on EARTH did you get the idea from that the north west of
Scotland gets strong sun on a regular basis?


Regards,
Nick Maclaren.

The message
from (Nick Maclaren) contains these words:

But where on EARTH did you get the idea from that the north west of
Scotland gets strong sun on a regular basis?

It does so: periods of more than an hour of continuous sun have been
recorded. Indeed, I got sunburn on top of Creag Meggidh in October in -
lessee - 1963. We were waiting for a herd of reds to move up the hill,
and they were content where they were. The back of my neck and the backs
of my knees were quite red that evening.

--
Tony
Replace solidi with dots to reply: tony/anson snailything zetnet/co/uk

http://www.users.zetnet.co.uk/hi-fi