|How do pigments absorb and reflect different wave
lengths of light?|
|Question Date: 2013-02-06|
Pigment molecules, just like any other
molecule, have electrons that can "occupy"
different energy levels. It turns out that in
pigment molecules, the differences between
certain energy levels correspond to the energies
associated with specific wavelengths of visible
light. Pigment molecules are often organic
molecules (they have carbon) with what are
called "highly conjugated rings" that allow
electrons to "bounce" around when they
get "excited." Many other pigments contain
transition metals, and actually transfer
electrons between multiple molecules.
But what does it mean for a molecule to "get
excited?" Broadly speaking, what this means is
that electromagnetic radiation (different kinds
of light such as UV, XRay, and visible light)
can hit a pigment molecule and confer energy to
the electrons in the molecule. If there is
enough energy transferred, the electrons
are "excited," or "go" to higher energy levels.
When the molecule's electrons "relax" back down
to its original or "ground state" energy, energy
is released in the form of visible light. I hope
There are many, *many* different kinds of
pigments in existence, and they are made of
different things. However, as for how they
Chemical bonds have energy levels, and these
energy levels correspond to different
frequencies of light. When a photon of the right
frequency comes along to move a chemical bond
from one energy level to another, the photon has
a good chance of being absorbed. If the photon
is of the wrong energy to move a bond between
two energy levels, either not enough to reach
the next level or too much so that it would
overshoot the next level, the photon will likely
not get absorbed.
I don't know how reflectivity works as well
as I do absorption (reflectivity is more
complicated), but I can tell you that the
process is similar in that electrons in
molecules are somewhat free to move and will
absorb, then re-emit, the photon of the same
energy. Metals are reflective because the
electrons in metals are almost free, and have an
infinitely large number of possible energy
levels, and thus can reflect any photon.
Most pigments work by absorbing certain
wavelengths of light. Other wavelengths are
reflected or scattered, which cause you to see
those colours. At the atomic level, certain
wavelengths of light are of the correct energy
to excite specific transitions of electrons in
the molecules or the solid. The energy of the
light is absorbed to excite the electrons, so it
is no longer able to be seen by your eye.
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