"Nick Maclaren" wrote in message
...

In article ,
"Franz Heymann" writes:
|
| There is no such thing as "focus of the glass structure". Each
light
| ray exits from the glass at the same angle as that a which it
entered.
| At worst, it might be displaced sideways parallel to its original
| trajectory by a millimetre or two. The intensity distribution is
then
| essentially the same as it would have been if there had been no
glass.

I am talking about reradiation from inside and, perhaps even more
importantly, other glass. Consider a greenhouse built like:

---
/ x \

A plant at point 'x' is, in some sense, at the focus of the
structure.

No. The intensity of reradiation by the glass which is received at
any point inside the greenhouse is proportional to the solid angle
subtended by the glass at that point. The value of this does not vary
all that much from point to point, and it is very small in comparison
with the incident radiation.
Radiation from the glass plays a minor role compared to convective
heat transfer. Remember that the temperature of the glass is only a
fraction of a percent different from that of any other object in the
greenhouse.

See below.

| Surely as time passes, all the objects in the enclosure will
try to
| achieve the same temperature?
|
| Er, no. That is FAR too simplistic a model.
|
| Err, no.

I suggest that you take a few max./min. thermometers, calibrate
them against each other, and place them at various parts of a
greenhouse (properly shaded from direct sunlight). They won't all
show the same values.

Of course not, to the extent that warm air is lighter than cooler air.
I have actually tried it, and there is not much to choose from point
to point, except for a small increase in temperature with height above
the floor


| Remember that there
| is an external source of energy, and therefore the most
elementary
| steady state calculations do not apply.
|
| I know that. I am almost right. The short term temperature of
each
| body in it will depend essentially only on its albedo. The
various
| plant leaves will have very nearly equal albedos.

Er, no. You have forgotten convection and evaporation. Those can
vary just as much as the albedo.

Touche. I was considering only radiation effects. That was obviously
wrong.

| I suggest you do that. Glass absorbs infrared quite strongly,
which
| is why infrared lenses have to be made of rather unusual
materials,
| many of which are in fact black as far as visible light is
concerned.

I know that. It isn't the point, which is the RELATIVE transmission
of near and far infrared.

You have lost me there.

| You seem to be unaware of the fact that there is a relationxsship
| between the reflection coefficient and the absorption coefficient
of
| any optical medium. A good absorber is a bad reflector, and
glass is
| a very good absorber of infrared radiation.

In physics, as in life, things are rarely in black and white. While
what you say is true, it does NOT have an albedo of one for infrared
radiation.

I did not say, or imply that.

| The greenhouse effect arises in fact because the glass absorbs
| essentially all the reradiated infrared quite close to the inner
| surface of the glass, whose temperature rises as a consequence.
Most
| of this heat is returned to the enclosed volume by convection and
| reradiation.

Ah. Cross-purposes. Yes, that is so. I was referring to the
'greenhouse effect', where reflection is more important.

That is not correct. The greenhouse gases *absorb* the infrared
radiation, they reflect little of it. The heat exchange effect is
still primarily a convective mechanism. Do think of the fact that the
temperature of the greenhouse gases will be less than around 90% of
the temperature at the surface of the earth. Consider what that means
in terms of relative amounts of radiated power per unit area.
You might spare a moment to have a read at the Wikipedia entry under
"Greenhouse effect".

It is possible that the "plants burning under glass" effect is more
due to reradiation from hot glass that reflection of the reradiated
infrared from plants.

Not so much of this "hot glass" lark! The temperature of the glass is
unlikely to be more than a degree or two above the ambient temperature
in the greenhouse.

I hadn't thought of that one. In particular,
it makes the 'focus of the structure' even more important.

No. There is no "focus of the structure".

I have spent a couple of hours playing with a shielded minimax
thermometer in a greenhouse. There were no noticeable hot spots
anywhere in it. There was only a gentle increase of temperature with
distance, amounting to about 2 deg. C between floor highest point.

Franz