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.
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.

| 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.

| 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 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.

| 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.

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


Regards,
Nick Maclaren.