Tim McNamara may have said:In article <eMdWa.33166$Ne.31813@fed1read03>,
Bill Davidson said:Bob Denton said:UV doesn't make you hot. It's infra red and visible light.
I'm pretty sure that's not true. Any time you absorb radiation, you get heat. In other words,
they all make you hot. It's possible that UV results in less heat but even that I doubt.
If all radiation absorption results in heating equally, why don't microwaves use UV frequency light
and use radio waves instead?
Light and microwaves of the same frequency are not the same thing. A microwave transmitter that is
operating on the same wavelength as visible light does not produce light. (Although the results of
the original experiment demonstrating this of which I'm aware were not published due to the nature
of the research facility in which they were carried out at the time, the result was not unexpected;
no physicist that I've told about it was surprised, and the average reaction was of the "Well, duh!"
variety.) A microwave transmitter may produce electromagnetic radiation at light frequencies, but it
will not produce light. If this were not the case, a "light receiver" could be tuned to a convenient
frequency and used for solar radiation collection with incredible efficiency. Unfortunately, it
doesn't work, because light isn't electromagnetic. (Sadly, Popular Science, high school science
texts, and the writers of that caliber have yet to come to grips with the distinction.)
Quoted message said:Quoted message said:Why does UV filtering (tint) on windows reduce the heat buildup inside a car or building; even if
it's a very light tint?
I suspect- but don't know- that heat gain through a transparent surface such as window glass, with
subsequent heating of the air volume enclosed by the windows, is a matter of different wavelengths
than is the case with skin.
Any incident photonic radiation which reaches the interior of a car, regardless of its wavelength,
may be absorbed and converted to heat, just as with light hitting skin; a car's interior, however,
has much less ability to shed that heat unless the car's engine is running and the A/C is operating,
while skin can sweat. Most of the light that gets to the surface of the planet isn't UV, so the UV
component is not the major player in car interior heating or skin heating, but UV (as you noted) has
other tricks to play with skin.
Most window glass will block some UV, but usually far from all of it. A photographic UV filter looks
like just a piece of clear glass, but it blocks 100% of UV. Visibly tinted glass doesn't necessarily
block UV, but it reduces the amount of visible light that gets through, thus reducing the amount of
heat produced by the light-absorptive surfaces inside. The light that is blocked at the glass
translates largely into heat that is shed back to the environment on the outside of the glass, and
thus does not significantly contribute to interior heating. The majority of the light which gets
through the glass is absorbed by, and therefore heats, the surfaces inside. Some of the light is
reflected by the interior surfaces; if the reflection is back out of the window, the heat load is
not increased. If it's reflected to another interior surface, the light may still contribute to
interior heating. In being reflected, of course, there is some heat produced since the reflection is
never 100%.
Quoted message said:UV tinting is primarily used to prevent fading of upholstery and doesn't- by itself- result in much
reduction of solar (heat) gain because the other frequencies are selectively unfiltered.
UV tinting would be invisible. Visible tinting isn't necessarily UV tinting. Many forms of glass are
inherently UV-blocking, but they're not commonly used as window glass. Much of the
commercially-produced window tint film now blocks UV entirely and visible light partially, but this
varies from product to product. I have seen UV-only window film, but although it's clear, it causes
visual distortion.
Quoted message said:We probably see the small portion of the electromagentic spectrum that we do because a broader
spectrum could not be focused on the retina as well.
Essentially correct, except that we see light, not e-m. The light spectrum extends well above and
below the frequencies we see, but it has a limited range. We are not immune to e-m; it can cause
damage, and heating, but we can't *see* the e-m radiation. (The degree to which e-m radiation can be
dangerous is familiar to any military pilot, at the very least.)
Quoted message said:Even within the range of frequencies we can visually perceive, the focal length for the short end
of the visible spectrum is slightly different than the focal length for the long end.
Yes. If you've ever used a high-powered microscope and noticed that the edges of the objects have a
chromatic tint, red on one side and blue on the other, that's caused by the focal length problem;
using a narrow-band fluorescent light source, or using a colored filter to restrict the light to a
specific frequency, improves microscope resolution somewhat.
Quoted message said:As far as sunscreen making one feel hotter- my subjective impression is that I feel hotter with
sunscreen on. I suspect this is due to the oil base that most sunscreens use, which reduces the
"wind chill" effect and puts an insulating layer between the skin and the sweat as it evaporates.
My skin is certainly much wetter with sweat when I wear sunscreen.
I've seen that as well, although I suspect that some of it may be that people tend to use sunscreen
when in locales where they're going to sweat a lot, and when engaged in activities which are likely
to increase the sweating. Non-UV-blocking suntan lotion often seems to have the same effect. Having
grown up in a tropical climate, I have a long-standing appreciation of the lack of a real substitute
for shade.
For biking in full daylight, particularly in summer midday conditions, I tend to favor as much
coverage as I can stand, and then apply sunscreen where the coverage is impractical. By preference,
however, I go out in early morning or late afternoon when the routes tend to be shaded.
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