Some crystals change color when exposed to light due to phenomena such as photochromism or thermochromism. These processes alter the structure of the crystals, resulting in a noticeable color change.
When light touches a crystal, certain electrons absorb specific colors (certain wavelengths, that is). They capture the light energy and jump to a higher level, entering an unstable excited state. Shortly after, they fall back to their initial level and emit new light. This back-and-forth absorption-emission gives the crystal its characteristic color. What it absorbs is not visible, and what it emits is precisely what our eyes perceive. The result directly depends on the internal atomic structure of the crystal: each crystal thus has its own energy levels, much like a unique optical fingerprint.
Crystals are like super regular networks, where each atom has a specific place. This organization influences how light passes through the material and determines the perceived color. When the crystalline structure undergoes a change, such as mechanical stress or exposure to intense light, the precise arrangement can shift slightly. This small change is sometimes enough to modify the energy bands, which decide which colors of light are absorbed or reflected. As a result, a true variety of different colors can suddenly appear. Even the slightest movement of atoms is enough to change how light behaves in the crystal; in short, it is ultra-sensitive!
Photochromism is the ability of certain materials to change color when exposed to light and then gently return to their original state once in the dark. Specifically, under the influence of light, the molecular structure of the crystal shifts slightly: the molecules temporarily change shape or arrangement, creating a new configuration that absorbs and reflects different colors. When the light disappears, the molecules naturally, but slowly, return to their original form, much like an elastic band that is gently released. This is called a reversible reaction because everything eventually goes back to how it was before. Several crystals and glasses, such as those in "magic" glasses that darken in the sun, use this amazing mechanism in everyday life.
Pure crystals generally have a color defined by their very structure, but add a few impurities and you get some pretty cool changes! These small doses of foreign atoms sometimes create intermediate energy levels in the crystal: the absorbed or emitted light can thus change wavelength, altering its color. For example, chromium in corundum transforms a clear stone into a bright red ruby, while a hint of iron will yield a blue sapphire. Sometimes, this is enough to bring completely unexpected hues to certain minerals. This process is called doping, and it doesn't take much: just a few atoms are enough to radically change the appearance of the crystal.
When a crystal is heated, the vibrations of the atoms increase. This agitation slightly modifies the distance between the atoms and subtly deforms the crystal lattices. The result: the optical properties also change. Sometimes, a small increase in temperature is enough to visibly change the color of the crystal. Another common effect: at higher temperatures, some crystals absorb or reflect certain wavelengths of light better, which alters their transparency or luminescence. In short, in the physics of crystals, temperature is somewhat like a color thermostat.
There is a particular crystal, alexandrite, capable of radically changing color depending on the type of light under which it is observed: it appears green in daylight but reveals a red hue under artificial light. This fascinating phenomenon is known as the alexandrite effect or color change by metamerism.
Did you know that the emerald gets its incredible green color from trace amounts of chromium or vanadium incorporated into its crystal structure? Without these valuable impurities, the mineral beryl would be perfectly colorless.
The famous purple amethyst loses its characteristic hue when heated to high temperatures, turning yellow or orange (citrine). This phenomenon is a direct demonstration of the essential role that thermal conditions play in the color of certain crystals.
Photochromism, which allows certain sunglasses to automatically adjust to light intensity, relies on specific crystals that are capable of altering their transparency under the action of ultraviolet light.
Yes, temperature can affect the optical reactivity of photochromic crystals by altering the dynamics of the atoms or molecules involved. Thus, some crystals exhibit more or less pronounced photochromic effects depending on the ambient temperature, which can even cancel out or intensify this effect under certain thermal conditions.
No, only certain crystals have the property called "photochromism." These materials have the specific ability to absorb and emit photons in a reversible manner, resulting in visible color changes when exposed to light.
The color of a crystal directly depends on the interactions between light and its atoms, as well as the crystalline structure. Differences in chemical composition, the degree of purity, or the presence of impurities alter these interactions, resulting in several possible colors for the same type of crystal.
Photochromic materials are widely used in adaptive eyewear, known as photochromic glasses, which darken in sunlight. They are also used as optical sensors, in rewritable optical memory elements, and in certain indicator panels and smart displays.
It depends on the mechanism involved. In the case of photochromism, the change is generally temporary and reversible; when the light source is removed, the crystal usually returns to its original color within a few minutes or hours. However, in some cases, due to an irreversible alteration of their structure, the crystals retain a new coloration for a longer period.
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