### Re: HOw does color affect heat absorbtion?

Date: Sat Jan 22 21:51:31 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 948156576.Ph
Message:
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It seems to me that the key piece of information you need is the equation
f = E / h
which was created by the physicist Max Planck, and published in the year
1900.  This equation was the starting point for the whole of Quantum
Mechanics (Happy Centenary!), but fortunately the thing it describes is
pretty simple:
(Frequency) equals (Energy) divided by (Planck's Constant).
(NOTES: Usually the equation is expressed as
(Energy) equals (Planck's Constant) multiplied by (Frequency),
but because of the lack of a unique and recognized-by-all multiplication
symbol, I have opted for the divisive form presented above.  Also,
Frequency is usually symbolized by one of the letters of the Greek
alphabet (nu), rather than the  f  above.  But my simple text editor
doesn't do Greek, so....)

We are talking about the frequency and energy of a single photon of light.
Frequency is measured in Hertz (cycles per second), and is directly
associated with the color of light.  Next, energy (when associated with
photons) tends to be measured in terms of 'electron-volts', which is the
amount of energy acquired by one electron as it accelerates across a
distance of one centimeter, under the influence of one volt.  This is a
very small unit; an average photon of visible light carries roughly two
electron-volts of energy (and radio-wave photons carry miniscule fractions
of that).  The most commonly used unit of energy larger than an
electron-volt is the erg (the energy needed to accelerate one gram of mass
such that its velocity changes by one centimeter per second).  The erg is
vastly larger than the electron-volt: one erg equals 6.242x10E11 (or
624200000000) electron-volts.  Finally, there is Planck's Constant; it is
simply 6.626x10E-27 (or 0.000000000000000000000000006626) erg-seconds.  It
is inherently just a description of how finely divisible is the "essence"
of the Universe; a different value than the one it has could be associated
with a radically different Universe than the one know.  The very small
value of Planck's Constant means we inhabit a very fine Universe (pun
intended; a 'coarse' Universe likely would truly be a rough place for us
to try to survive!).

To be mathematically consistent, the measurement-units associated with
frequency, energy, and Planck's Constant must match; thus I offer this
minor conversion:  Planck's Constant is 4.136x10E-15 (or
0.000000000000004136) electon-volt-seconds.  From this you can see that
even for an electron, Planck's Constant describes a fine state of affairs.

-------------------------------

Your experiment is going to provide you with a wealth of research
opportunities in a library (or on the Web).  For starters, you will need
data on the output of your sun lamp, in terms of how much energy is
emitted at each color-frequency that you will be testing.  (One source for
such data is typically the manufacturer.)  It would do your experiment no
good to fail to take into account the fact that most incandescent light
sources emit significantly less blue light than they emit yellow light.
Then, for each frequency, this energy should be converted from, say, ergs
per second into photons per second.  Then, from the angles at which light
leaves the sun lamp and reaches the liquid-containers, you must figure
what portion of the photons actually arrive....

Next, you will need data on the exact spectral absorption signatures of
each of the colored filters that you will be using.  A filter that
transmits red light generally does not transmit 100% of all frequencies
(or even 100% of any single frequency) of red light, for example.  But
once you have the data, you can then compute how many photons per second
of each freqency pass through the filter and into the liquid.

You will need similar absorption data for each of the liquids you plan to
test.  Keep in mind that since the liquids you mentioned (water, alcohol,
and salt water) are pretty transparent, you may have to operate your
experiment for a lengthy period in order to be able to measure the
results.  The PURPOSE of gathering all the preceding information is to
become able to predict the results.  It is by comparing predictions to
actual measurements that Science ultimately progresses, because when the
results don't match, that means something new is waiting to be learned.

Other factors to be wary of are:  The background upon which the beakers
rest; will it absorb light, grow warm, and directly influence the
temperatures of the liquids? -- or will it reflect light, and thereby
change the number of photons that you would expect to pass into the
liquids?  (Perhaps the beakers should be suspended away from any
background.)  What of the temperature of the surrounding air; how will its
absorption of light from the sun-lamp affect the beakers?  And what of the
fact that alcohol (and to a lesser extent water) tends to evaporate
easily?  Your beakers may have to be sealed! -- since the experiment's
duration may lead to less liquid being heated than you planned.

There may be more things to keep track of than I have mentioned here.  The
longer the list, the more difficult the experiment -- but the greater the
reward will be in a science fair.  I wish you the best of patience, skill,
care -- and luck.

```

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