MadSci Network: Physics

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

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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