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Changing Colors of Materials
Name: Nicolas
Status: student
Grade: 6-8
Location: N/A
Question:
Is there any method to change the reflection
characteristics (I mean the color) of a solid?
Replies:
Nicolas,
Try this at home. In a pool of water (possibly in a bath tub, a swimming
pool, or even a deep pan of water) create a wave that is directed toward
the wall of the tub, pool or container at an angle of approximately 45 degrees.
If you do this gently enough so that you can see the wave travel toward the
wall and bounce off the wall, you will see that the wave manages to form from
the wall and continue to travel in the reverse direction from where you
started the wave. Now, place a stick in the water and push a wave toward it.
The wave will not reflect off the stick. You might see the wave break around
the stick and seem to form different patterns.
In the same way, light bounces off solids so that the wave (after certain
colors have been absorbed) are reflected off and we see particular colors.
However, if the surface of the object is much too small compared to the size
of the wave then no reflection occurs, the light is scattered. As such, since
no light -in the visible range- reaches our eyes, the object will appear black.
If iron filings are available to you (and they are small enough in size),
compare the color of the iron filings with a block of iron, I think you will
see that they have a different reflective characteristics.
Greg (Roberto Gregorius)
One way to answer the question is "No" because if you do not change the
substance at all, then whatever you do to it will not change either. But that is
a trivial answer.
An equally trivial answer is to cover the object. One way is to paint it. In
fact, there are lots of 'coatings' or 'thin films' you can put on solids
that will change their color. Also probably not what you were looking for.
A third way is to chemically change the solid -- for instance iron will rust
and turn from black to, well, rust-colored. Or to add something to the solid
(rubies and sapphires are similar chemically, but differ by trace
impurities). The chemical differences are small, but the color difference is
huge.
In a way, this is just like mixing in a dye -- you add something and the
color changes. You might not like these methods because they're "cheating"
by covering or changing the material. So maybe a different question could
be, "Can we change a substance's color without adding or removing anything?"
One option is to rearrange the atoms in the substance. Titanium dioxide
(TiO2) is a substance make of titanium and oxygen. However, you can arrange
the Titanium and Oxygen in different ways to get different looking
substances. "Anatase", "Rutile", and "Brookite" are all the same atoms, but
in different arrangements, and they are not the same color. But this idea
is not the most useful because you cannot just move around atoms with your
hands. It is quite challenging to take a sample of one of these materials
and end up with another.
So the last option is to change its size. You may know that light waves are
about 500nm in wavelength. When you start dealing with things that around
this size (or smaller), you can get some weird effects. For example, an oil
slick (a very thin layer of oil on water) can look rainbow-colored even
though a cup of the oil looks black. It is because the layer of oil is thin
enough that light bounces off of it in a unique way that causes the rainbow
pattern (it is called 'interference' -- you can look it up). Another way is
to make the substance very, very small. When you make very small particles,
just a few or several dozen atoms big, their size can determine their color.
Of course, if you take a substance and break it into nanometer-sized
particles, you have still changed it quite a bit.
So as you can see, there are many kinds of changes, but to change the
substance's color, you have to change it somehow. And if you change it, it is
no longer the same.
Hope this helps,
Burr Zimmerman
Hi Nicolas,
There is nothing that I am aware that can change the inherent
reflectivity characteristics (especially color) a solid. Color is
determined by the interaction between photons, a material's outer
electron shells, and the arrangement of its atoms or molecules. To be
sure, there are surface treatments that affect reflectivity, and also
cause the material to take on different colors, but this is not a
change to the material itself. There are also sometimes some surprises
when a material is made thin enough. As an example, gold, when rolled
into thin sheets only a few atoms thick, starts to take on a greenish
hue. Other metals generally do not behave this way, but that is mainly
because no other metal can be rolled into sheets as thin as gold can.
The apparent change in color when gold is made thin enough, is only an
effect caused by its extreme thinness.
Another example is carbon which can either appear black (as graphite)
or clear (as diamond). But this is only caused by diamond and graphite
having entirely different crystal structures. Phosphorous too, can
appear white or red, depending on which of two natural crystal
structures (so called allotropes) it has. For any material with a
specific crystal structure, its color cannot be changed.
Regards,
Bob Wilson
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Update: June 2012
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