jtb@phs.UUCP (11/15/83)
In reply to your question about the reflection of heat: All electro-magnetic radiation carries energy we call infra-red light "heat" because we can feel the heat when it strikes our skin but we can't see it. Actually on a per photon basis visible light has more energy than infra-red. The energy traping which occurs in a room with a window or a planet with an atmosphere occurs because some of the sun's light is absorbed and the energy is re radiated as long wave infra-red if the glass or atmos- phere is not transparent to this infra-red the energy will be traped. That fraction of the light which is reflected however will pass out of the window or atmosphere as easily as it came in. Since most of the sun's energy is in visible light or near(short wave)-infra red the visual reflectance of a surface is critical in determing how much energy it will absorb. Jose Torre-Bueno decvax!duke!phs!jtb
REM%MIT-MC@sri-unix.UUCP (11/23/83)
From: Robert Elton Maas <REM @ MIT-MC>
Date: 15 Nov 83 10:00:46-PST (Tue)
From: decvax!duke!phs!jtb @ Ucb-Vax
All electro-magnetic radiation carries energy we call infra-red light
"heat" because we can feel the heat when it strikes our skin but we can't
see it.
This is a matter of terminology (as contrasted with fact), but I
believe the accepted terminology is that only energy with a wavelength
longer than red yet shorter than microwave is considered infra-red,
despite the fact that all energy absorbed by the skin will warm it to
some extent. Thus all electromagnetic radiation carries energy, but it
isn't called infra-red unless it's in the correct wavelength band. For
example, spectrally-pure light from a visible-light laser contains NO
infra-red whatsoever (of course any practical laser emits some thermal
radiation in all directions, some of which happens to be going in the
same direction as the laser light, but the coherent light carries NO
infra-red at all). Thus I think your statement is wrong/misleading.
The rest of your explanation is reasonable, so let me offer a
replacement for the erroneous part:
All electromagnetic radiation contains energy, and when absorbed by an
object causes it to get warmer. Warm objects then emit electromagnetic
radiation of all wavelengths (not just the wavelength of the incoming
energy that heated it in the first place), the so-called "black-body
radiation". At Earth temperatures, most of the radiation given off is
in the infra-red, and planetary atmospheres tend to block infra-red
even if they allow light to come in, allowing the planet to get warm
from the light but making it difficult to cool back off by emitting
infra-red, causing the planet to be warmer than if it was exposed to
space directly where it could both absorb light and emit infra-red freely.
P.s. this blocking of infra-red radiation, causing the inside (the
planet in this case) to get warmer, is called the "greenhouse effect"
because the first practical use (other than the natural environment of
the Earth and Venus etc.) was in botanical greenhouses, which can grow
plants comfortably-warm inside even when it's unconfortably cold
outside and there's no furnace or other heat source inside the greenhouse.jtb@phs.UUCP (11/28/83)
What I said about infra-red being called "heat" was meant to explain how the common usage came about not to say that was correct usage. Of course all absorbed radiation will heat an object it's just that visible light and infra-red are the two forms of EM radiation that are most common in everyday life. Jose Torre-Bueno decvax!duke!phs!jtb