Anomaly Magnetism said:
E-M radiation at infrared frequencies?
E-m radiation from a thermal source is broad-band and contains a whole range
of frequencies. The 'peak' of the distribution is a function of the body's
temperature and is often in the infra-red but not always. Take for example
the Sun. It radiates a wide range, the 'peak' distribution is in the
'visible' light range of frequencies (evolutionary biologists might argue
that the 'visible range' is only called that because we evolved eyes to use
the most prevalent frequencies available from the Sun, but that's another
story ;-)
This is confusing. So many science books say that infrared radiation
is heat radiation.
That is a simplification. Infra-red is e-m radiation, like any other except
it is considered to be of a particular range of frequencies. In early
works, each 'band' of e-m radiation was given a name by the folks that
worked with it the most. 'Radio' waves, 'infra-red', 'visible', 'cosmic'
are all names for e-m radiation of a general frequency range. Certain
things about all of them are the same, other aspects change as the frequency
of the e-m changes. All travel through vacumn, all are composed of
alternating electric and magnetic fields at right angles. All show
properties similar to 'waves' and 'particles' (called photons with a 't').
But different frequencies carry different amounts of energy for each
'particle'. The way each frequency interacts with matter is somewhat
different. Some frequencies pass through most matter with attenuation
determined by the density of the matter (x-rays). Other very narrow
frequencies are reflected from objects to your eye giving different objects
a different color. ('reflection' is even a simplification, but lets not get
*too* complicated here)
Yet, medical journals dealing with radiation injury
state that exposure to higher frequencies results in further
concentration of generated heat in the surface of the skin, while
lower-frequencies allow generated heat to dissapate faster.
Higher-frequencies have a poorer Dissapation-to-Generation Ratio (DGR)
than lower-frequencies.
Higher frequency e-m radiation carries more energy per photon. In the range
of infra-red, this energy simply raises the temperature of the molecule it
interacts with.
Now, if you have two sources radiating e-m in a broad spectrum as thermal
emitters, and one is much hotter than the other, the hotter one will be
radiating more higher-frequency photons.
I don't know your medical text on DGR, but it seems how far the radiation
travels *into* the tissue before interacting would be an important factor.
If all the radiation interacts within the first few millimeters, than all
the energy is deposited there. If the radiation doesn't interact as easily
with the compounds of skin (water, amino-acids, whatever), then the
radiation travels deeper and the energy is deposited in a larger volume of
tissue. The energy is then able to be distributed from that larger volume
more effectively before the temperature rises to the point of damage.
"DGR" is the amount of the heat dissapated versus the amount of heat
generated.
Given a constant amplitude (wattage), the overall amount of heat
produced is the same. However, due to the poor DGR of short-wave
radiation, burns associated with higher-frequencies often show further
thermal protein denaturation than burns associated lower-frequencies.
Given the same wattage, 'short-wave radiation' would contain more higher
energy photons. If the tissue is more 'opaque' to these higher energy
photons, they will deposit their energy in the top-most layers of tissue.
With the same energy being deposited in a smaller volume, its temperature
will rise higher, no doubt causing more injury.
But note that its the relative opacity and how much of the radiation that
interacts with the tissue that is important. X-rays are much higher energy
still than your 'short waves'. Yet much of the body is relatively
transparent to them and most pass through with little interaction. Not that
you should stand in front of a 100 Watt x-ray machine for as long as you
would in front of an 'infrared heater' ;-)
The other important point of e-m radiation and the human body is the exact
nature of the interaction. Infrared will raise the kinetic energy of
molecules (i.e. warm them). Radio and microwaves can excite water molecules
to a high degree (such as in microwave ovens). X-ray, gamma and cosmic
radiation can break the bonds holding molecules together causing
free-radicals and a *potential* for gene failure. So the same amount of e-m
energy aimed at the human body can have different effects depending on how
that energy interacts with the matter making up the human body. And if the
energy is deposited slowly over time, the body can repair the damage more
readily (a low chronic dose of gamma rays over years is less harmful than
one short massive dose).
Then I take it there is no way of measuring heat without the presence
of matter.
Actually, the point of the other discussion in this thread is whether you
can measure 'temperature' without matter. Measuring 'heat' is saying,
'measure the kinetic energy of molecules'. That is hard to do unless you
*have* molecules ;-) But it is also hard to measure the energy of e-m
radiation unless you cause it to interact with some matter and observe the
effects ;-) Even your own eyes wouldn't work unless the e-m radiation
(visible light) interacts with the structures in the retina.
daestrom
