Re: Spectrosopy
- From: "Ioannis" <morpheus@xxxxxxxxxxxx>
- Date: Tue, 29 May 2007 00:38:11 +0300
"Skywise" <into@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:135mdvfkd9g6h2a@xxxxxxxxxxxxxxxxxxxxx
I have what may seem to be a simple question.
I understand emission spectra. I also understand absorption
spectra. If you have an excited media, it will emit spectral
lines. Conversely, a media will absorb on those same spectral
lines any light transmitted through it.
But what about reflection? Say I shine a "white" light on something
and view the spectrum of the reflected light? Would I be looking
for absorbption spectra?
In general, yes. Light falling against opaque surfaces, suffers absorption.
The remaining components (those which are not absorbed) are reflected by the
surface back to the viewer.
The sum total of the reflected and the absorbed components will give you
exactly the light of the source.
To give a more specfic example, say I want to determine the
contents of a cloud of smoke by viewing the spectra of the
sunlight reflected off the cloud? Yes, I realize I have to
compensate for the Sun's spectra. But I'm not 100% if I'd
be looking for abosorption or emission lines.
In general when the medium is transparent (or semi-transparent or
transluscent) one looks at the absorption spectrum of the light passing
/through/ the medium. In the case of a cloud of smoke for example, you'd be
better off looking for the transmission absorption spectrum. It is exactly
this spectrum which is characteristic of the red/purple daytime sun or of the
yellow nightime moon which you see in cases of great forrest fires. The smoke
cloud basically absorbs a good part of the visible spectrum, so you are
getting an absorption spectrum.
Something similar happens on a cloudy day or with organic materials which are
examined by professional spctrographs. Hemospherin (and I think Hemoglobin,
but don't quote me) have characteristic molecular absorption spectra,
therefore one can get lots of info on blood samples by stuffing the sample in
a spectrograph. That's how most microbiologists do it. Same for urine, etc.
All that if you are lucky and the medium is transparent/transluscent. If the
medium is opaque, same rules apply, but one looks at the reflection absorption
spectrum. Many flowers/fruits absorb parts of the visible spectrum and reflect
only specific bands in which bees and birds are most sensitive.
The reflection absorption spectrum of paints for example is characteristic of
the paint color. For a magenta paint, you can be sure that the middle part of
the spectrum is absorbed and only the red and blue parts get reflected.
You can test if light of a particular wavelength gets reflected off an opaque
surface by looking at its color when the surface is illuminated by
monochromatic light. Try illuminating some material with a red laser diode,
which emits only at lambda=660nm and see what color you get. If it's similar
to the source color, it gets reflected. If the color is dumbed down or looks
brownish, it is absorbed. Something similar happens with low pressure sodium
lamps.
Things start getting hairy when you start illuminating substances with the
light produced by the same substance in an excited state.
All the above for example does not apply if you illuminate hot sodium vapor
WITH light from a low pressure sodium lamp. The light from the lamp causes the
atoms in the hot sodium vapor to resonate, as a result, you don't get an
absorption spectrum (whether from transmission or reflection) rather, you get
the emission spectrum of sodium FROM the illuminated gas as well.
For cases where the medium is both reflective and transmissive (such as with
anti-reflection coatings on optics and lenses) you can be sure that the R_p
and R_n components will be exactly complementary, at least in the visible. For
example, with my glasses which have a Hoya broadband ar coating, when one
looks at me face on, one sees a green tint on the lenses. This green is
precisely the integrated absorbed component, when you look at the lens
absorption-wise.
So, if you look at the /transmissive/ absorption spectrum of such a lens, the
reflected green component will be missing and will be showing as absorption
with a peak near the integrated color of the green tint. If you look at the
/reflective/ absorption spectrum of such a lens, the bits which will be
missing will be the components which make it through the lens.
That's why red-coated binoculars are trash for astronomy: Because the R_p
component is too strong and the coating reflects too much visible light, which
on the other end (transmission-wise) doesn't make it through, thus reducing
overall nightime brightness.
Any simple referrences that discuss this technique? (I am still
googling as I write this)
Google images for "reflection spectrum", and "absorbance spectrum".
Brian--
I.N. Galidakis --- http://ioannis.virtualcomposer2000.com/
----------------------------------------------------------
"There's ALWAYS a mistake somewhere"
.
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