sunscreen makes you hotter?

Tim McNamara wrote:

In article eMdWa.33166$Ne.31813@fed1read03,
Bill Davidson wrote:

Bob Denton wrote:
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?


For a photon:

E(f) = h_cross*f

So if the energy is indeed _absorbed_ (not reflected, or...?), then photon for photon, higher frequency does
mean more heating.

I seem to recall a belief that the water molecule was "resonant" at some frequencies. If so then coupling
energy into them may be more effective at some frequencies than others (both higher and lower than resonance
might be less effective). For example: 2.4 GHz. Sorry about the mights and maybes; that is all you get for
an expedient response that *may* not be accurate. I'm not a specialist there.

Werehatrack wrote:

On Thu, 31 Jul 2003 15:40:46 -0500, Tim McNamara
may have said:

In article eMdWa.33166$Ne.31813@fed1read03,
Bill Davidson wrote:

Bob Denton wrote:
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.


This is a nonsensical statement.

A
microwave transmitter that is operating on the same wavelength as
visible light does not produce light.

As was that. Microwave != visible light, by definition.

Microwave is defined: 300 MHz to 300 GHz. However, most microwave engineers don't seem to consider it
"microwave" until about 1 GHz, due to the fact that they can more or less get away with *point form*
components below 1 GHz as opposed to the more accurate but more unwieldy *distributed form* used as the
frequency climbs.


I suspect- but don't know- that heat gain through a transparent
surface such as window glass, ...

Anyone who can figure out that "heat gain" thing ought to patent it and become the richest human ever.

Any incident photonic radiation which reaches the interior of a car,
regardless of its wavelength, may be absorbed and converted to heat,...

This much is true. In essence, tinted windows reduce transmission (reflect or absorb) to the interior of
certain spectral components, so naturally the total photon energy reaching the inside of the car will be less
with tinted windows.

This is a joke, right?

In any case, light, microwaves, radio waves are all electromagnetic
radiation. The only difference is wavelength (or, if you prefer,
frequency). This is basic physics.

Solar energy is largely black-body radiation at about 5600K, which peaks
at a wavelength around 1 micron, in the infrared. Not that it
matters--the issue is cooling, not heating. We know that we get hot
while bicycling. What else do you need to know?

A thin layer of sunscreen provides a trivially insignificant amount of
insulation. Most of your cooling comes from evaporating sweat anyway,
not conductive heat transfer through your skin, so the effect of
sunscreen is negligible.

By the way, if you're really wondering why ovens use microwaves instead
of UV, aside from the fact that the UV would blind everyone, what would
you use for an economical source of 1500W UV energy?



Werehatrack wrote:



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.)

"Steve Maas" wrote in message
...
This is a joke, right?

In any case, light, microwaves, radio waves are all electromagnetic
radiation. The only difference is wavelength (or, if you prefer,
frequency). This is basic physics.


What if the biggest influence is that it interferes with your sweat and its
job in cooling your body? Would it still be a joke?

Solar energy is largely black-body radiation at about 5600K, which peaks
at a wavelength around 1 micron, in the infrared. Not that it
matters--the issue is cooling, not heating. We know that we get hot
while bicycling. What else do you need to know?

A thin layer of sunscreen provides a trivially insignificant amount of
insulation. Most of your cooling comes from evaporating sweat anyway,
not conductive heat transfer through your skin, so the effect of
sunscreen is negligible.

By the way, if you're really wondering why ovens use microwaves instead
of UV, aside from the fact that the UV would blind everyone, what would
you use for an economical source of 1500W UV energy?



Werehatrack wrote:



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.)

Steve Maas wrote:

My source [for solar radiation] is the curves in Radio Astronomy by
John Krauss. Take a look at them; I think you'll find that the peak
is fairly broad.

[Wish you wouldn't top post, it makes following a discussion
difficult]

Here's my most convenient source:

Roshenow et al, _Handbook of Heat Transfer_, 1998, p. 7.5:

"the solar radiation spectrum has a peak at about [lambda] = 0.5
[microns], as T(sun) = 5762 k"

Those curves you are looking at are probably on a log scale, making
the distribution look wider than it is. Using the Planck
distribution equation, the sun's energy at 1 micron is only 41% of
its energy at 0.5 micron. Not a big difference on a log scale, but a
substantial difference in energy.
--
terry morse Palo Alto, CA http://www.terrymorse.com/bike/

save the technobabble...

as a strawberry blonde who relies heavily on his sunscreen i absolutely
guarantee you that sunscreen is less hot than a 1st or 2nd degree burn.

and while i'm spouting...the new alchohol-based 'gel' formulas absoultely
blow the old lotion-style formulas away in terms of comfort and ease of
application.

$0.02,

dookie
(whose sworn brand is bullfrog quickgel)

"Steve Maas" wrote in message
...
My source is the curves in Radio Astronomy by John Krauss. Take a look
at them; I think you'll find that the peak is fairly broad.


Terry Morse wrote:

Steve Maas wrote:


Solar energy is largely black-body radiation at about 5600K, which peaks
at a wavelength around 1 micron, in the infrared.


You're off by a factor of two. Solar energy peaks at about 500 nm,
which is visible yellow/green(*). The percentage of solar energy in
the infrared range is rather small when compared to the visible
light it emits.

* Wien's Law: peak wavelength = 2.898E-3 / T
--
terry morse Palo Alto, CA http://www.terrymorse.com/bike/