Ask A Scientist© Chemistry Archive

name         Daniel F.
age          30s

Question -   How can I determine what the true color of visible
light is of a gas cloud?
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Example: Visible light falls between 400nm and 750 nm,
{violet 400-424, blue 424-491, green 491-570,
yellow 570-585, orange 585-647, red 647-700}

A cloud made up of say sulfur gas gives us a spectrum
range of 4000 to 5500 Ångstroms. Sulfur is yellow in appearance
and I assume a sulfur cloud would also be yellow in some form of hue.
Making a sulfur cloud in the visible light area around
570nm-585nm. How can I convert 4000 to 5500 Ångstroms to 570nm-585nm?

What is the formula or conversion? Am I even asking the right
question? Could I use the atomic wieght of sulfur to determine what
it's color should be?  Or would some other fact on the periodic table be
able to give me an easier way to find the color? I know that not all
atoms have a frequency that can relate in this manner (ie: hydrogen)
However H2O has a property to refract light givingit a blue tint.


Ideally I would like to be able to use a fact (easiely found) about an
atom, or molecule, such as it's atomic wieght and run it threw a formula
to determine its true color (400nm-700/750nm) Is there such a formula?

You are asking two questions.  I will answer the hard one first.  You ask if
there is a simple formula to determine the color of a molecular species.
Well, it can be calculated, but it is not simple.  Basically, it involves
calculating the "wave functions" and energies of the highest-energy
electrons in the molecule, and of the molecule in an excited electronic
state.  This requires an awful lot of computation.

The other question you seem to be asking is the conversion factor between
angstroms and nanometers.  That one is easy.  There are ten angstroms in a
nanometer.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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This is really several questions, so let us start at the very beginning. A
substance will have "color" if it SELECTIVELY absorbs, reflects, or scatters
light between the wavelengths of about 400-700 nm, as well as any
combinations of the these processes. It must do so sufficiently to be
perceived by the eye, or some light measuring instrument. A "spectometer" is
such an instrument that spreads the wavelengths of visible light out
geometrically [like a rainbow does to sunlight] and focuses wavelength
selectively on a detector -- either photographic or electronic.

What makes the substance absorb, reflect, or scatter light can be very
different, depending upon the substance, its physical state [gas, solid,
liquid], and many other variables, so the question is not: "What is the true
color?" There is no "true" color, it depends on many factors. Some examples
may help you see what I mean.

Carbon in the form of graphite is black -- it absorbs essentially all the
light that strikes it -- but the same carbon in the form of diamond is
completely clear, meaning it transmits essentially all the light that
strikes it. In both cases I am neglecting some side effects like a little
reflection or refraction.

Oxygen gas, for all practical purposes is colorless [ the sky is blue
because of light scattering not the absorption of light by oxygen ].
However, liquid oxygen is beautiful pale blue, and liquid ozone [O3, another
form of oxygen is deep blue].

The elements F2, Cl2, Br2, I2 range in color from pale yellow to dark red as
solids, liquids or gases.

Compounds of the metals in the middle of the Periodic Table [ called the
transition metals -- Fe, Ni, Co, Cu, etc. ] have many different colors
depending upon the specific composition of the compound they are in.

There just is no simple way to predict with any certainty what the color of
a particular chemical compound, except for some educated expectations based
upon experience.

Vince Calder
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Sorry, there is no simple formula that will allow you to predict the 
spectrum of
an element or compound.

To convert from Ångstroms to nm simply divide by 10 (i.e., 5500 Ångstroms is
equivalent to 550 nm).

In the sulphur example you have given the absorption spectrum of sulphur 
is 4000
to 5500 Ångstroms or 400 to 550 nm.  This falls in the violet, blue, and green
regions of visible light.  This is the light that is ABSORBED.  What is left
over, and thus can reach our eyes, is the yellow, orange, and red wavelengths.
The mix of these remaining wavelengths is such that the overall appearance is
yellow.

  Greg Bradburn
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