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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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