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Jeffrey willamson tree and small fruit production. He has received grants
from both the National Institute of Standards and Technology and the Department of
Agriculture. Most of his professional work has been with NIST, including research
on the National Institute of Standards and Technology's Advanced Research Projects
Agency's National Measurement System, which is the basis of the United States
metrology infrastructure and the United States measurement system in general.
He has done extensive work on the Measurement of Optical Reflectivity and Interference
Spectroscopy, also known as reflectometry, which allows the quantitative determination
of the optical thickness of thin layers of various materials, particularly semiconductor
devices such as solar cells.
reflectance of a material, e.g., its reflection coefficient, is a fundamental parameter
in the quantitative study of surface properties of materials, including their surface
resistance and scattering. As applied to thin films, it is a powerful tool for characterizing
the optical properties of materials.
A more detailed explanation of reflectance measurements can be found here.
nothing of the physics of this, but my intuition tells me that the reason
for this is that photons coming from the white light source
are being absorbed by the sample, and the white light reflected is
some combination of the photons that had been absorbed and were
emitted at longer wavelengths.
The amount of white light absorbed is related to the wavelength
of the light. The shorter wavelengths are absorbed more, but are
less energetic, and so are less effective at causing chemical
reactions in the sample.
If you increase the power of the white light source to a higher
level, photons with shorter wavelengths are absorbed even more
frequently, resulting in the sample changing more quickly.
I hope that makes some sense. Feel free to tell me otherwise.
On the other hand, the white light is coming from an incandescent
source. The lower the temperature of the incandescent source, the
shorter the wavelength of the light it is emitting.
If you look closely at an incandescent bulb, you will see that
the inside surface of the bulb is rather close to black.
So, if you shine a light at this black surface, you are not going
to be much affected by the wavelength of the light.
The reason why I bring up the incandescent bulb here is that,
if you look at the graph in your reference, it shows the
percentage of light energy absorbed at various wavelengths
as a function of the wavelength. (See figure 10.7 on page
Incandescent bulbs are around 800-1000 nanometers
in wavelength, which is much too long to be absorbed by the
sample. So, if the wavelength of the light is too long to be
absorbed, there won't be enough energy to cause chemical
On the other hand, if the wavelength of the light is much too
short, like that of a blue light, the energy is not enough to
cause the chemical reaction either. But at least we know where
the problem is!
You will find that most of the energy of the source will be at
wavelengths longer than 3000 nanometers. Since our white light
source emits around 4000 nanometers, that means a large fraction
of the light energy will not be absorbed by the sample.
Let's examine the graph in more detail. Look at the left part
of the graph where the light intensity is 1. You can see that
more than a third of the energy is above 3000 nanometers and
therefore not able to cause any chemical reactions. Look at the
right part of the graph where the light intensity is more than
200. You can see that the energy above 3000 nanometers is even
worse than in the left part, and that's bad news!
So, if you have a white light source, you have to use
filters to eliminate the energy above 3000 nanometers.
When you go to look at the absorption spectrum of the solution,
you will see peaks at a number of wavelengths. At these peaks,
there is a lot of energy at that wavelength. So, this light
source will also cause lots of chemical reactions in the sample.
To figure out how much energy is absorbed at a given wavelength
x, you take the power at the wavelength x and multiply it by
the absorption cross section at that wavelength. The absorption
cross section is the probability that the light is absorbed by a
particle. If you think about it, the light has a certain amount
of energy, and it has to have more energy than the particle, so
the light can either hit the particle or can go around it. The
light will only hit the particle if the particle is large enough
to absorb the energy. That's the cross section.
The next thing to do is to take the concentration of the
particle, which is called the molar absorptivity of the particle,
and multiply it by the extinction coefficient, which is the
ability of the molecule to absorb light.
Now, to summarize all of this: To figure out what is going on
in a sample, you will need to look at the extinction spectrum,
and this is done by shining light at the sample, and looking at
the amount of light that is absorbed. This is called the
absorption spectrum. The shape of this absorption spectrum
tells you how the molecules in the sample absorb the light.
There are three things that are important:
The total amount of light that is absorbed
The wavelength of the light that is absorbed
The amount of light at a given wavelength
There is more information here, but this is all that you need
to know to start analyzing chemicals in a