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Name: Joshua T.
Status: other
Grade: other
Location: VA
Country: N/A
Date: 4/11/2005


Question:
What is the physical property of an object or substance that produces its color? I understand the process of light reflection and absorption, but what exactly about the object causes light to reflect at a certain wavelength?


Replies:
Usually it is the converse, Joshua. The absorptions are selective. The reflection is broad-band.

Flat-colored objects try to reflect light over the whole visible spectrum, but that reflection is distributed over a surface layer of some depth, and may be preempted by a strong absorption within that depth.

Imagine clear resin packed full of sparkly but very fine sand.

If there is no absorption (i.e., the resin is perfectly clear), the object is white. The object is filled with trapped light bouncing around amongst the sand particles. (It partly reflects off each surface of each grain, because the clear sand has a different index of refraction than the clear resin.) The sand eventually reflects all the light back out.

As one adds absorption at all wavelengths (i.e., the resin is dyed clear but dark like smoky-quartz), the object fades to gray then black. There is a competition between the "scattering length" and the "absorption length". Whichever is shorter dominates. Percentage splits like gray happen when the two lengths are similar.

Often an absorption is resonant, frequency-selective, like a clear-color in the resin. A pale purple dye absorbs green most, so the red and blue make it through stronger. Red light +Blue light = perceived color magenta. At the selective absorption wavelength, less of the light survives until it escapes to be seen by the eye. The wavelengths less absorbed then dominate the color we perceive. Filled with sand, it would become a flat purplish-pink. Increasing the concentration of the purple dye, it could become darker flat purple.

Color-selective reflections happen too, but are less common and tend to look special to our eyes. Things like opals, mother-of-pearl, and many iridescent animal colors are caused by periodic ripples in the index of refraction within an otherwise clear substance. One faint index-variation (ripple) reflects very little light, but if there are many successive ripples with regular spacing, the reflections add up favorably for wavelengths having one wave-length difference between successive reflections. That tends to mean the ripples are some multiple of half a wavelength apart. However the colors tend to change with viewing angle, because the lengths of the light-ray paths change with viewing angle.

Metallic colors (meaning shiny blue aluminum, not metal-flake paint) have only one reflection, that at the silvery surface of the metal. Over the metal is a colored clear-coat of any convenient substance. Naturally, if light goes through the clear-coat at a shallow angle, it is traveling farther in the absorbing medium, and more of it is absorbed. This is part of why foxy auto paint-jobs sometimes look darker around the edges and glow a little brighter in the middle. (The other part is that the painter may have applied different thickness on different parts of the surface.)

Jim Swenson


The subject of light reflection, transmission, and absorption is a very involved area, but some ideas are straightforward.

The best question is "Why is the sky blue? The answer is that molecules of air have resonances in the ultraviolet. This means that shorter wavelength light, which is bluish, is more strongly absorbed and then re-emitted than light of longer wavelengths, which is reddish. So when you look at the sky, you see mainly the blue light which is scattered out of the sunlight passing through the atmosphere while the red light passes through relatively undisturbed. This also explains why sunsets tend to be red.

Likewise if the molecules of some material tend to scatter (reflect) green light strongly and absorb other wavelengths, the material will look green. Similarly if all wavelengths are roughly equally reflected, the material will look white; if all visible wavelengths are strongly absorbed, the object will appear black.

Another phenomena is illustrated by water which mildly absorbs light of red wavelengths; it takes about 100 feet of water to absorb essentially all the red light. This gives large bodies of water a very light blue-green tint

Dick Plano, Professor of Physics emeritus, Rutgers University


Joshua,

Each atom and molecule has specific energy levels. When a single unit of light, called a photon, hits an atom, it is either absorbed or not absorbed. If the photon has the right amount of energy in it, the atom can absorb it and go to a higher energy level. This is usually an electron passing into a higher orbit. This light energy then becomes heat energy within the material: it is never seen again. If the light is almost the right energy, it can be absorbed but not held. This light is reflected.

For a photon of light, energy is proportional to frequency. Frequency determines color. Blue photons have more energy than red photons. This is why the very hot flames emit blue light. If you shine light on a red object, the molecules can hold green and blue light, but not red. The red gets absorbed, but then released back into the world. The green and blue photons stay in the molecules, making the material warmer.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College



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