#june25posts

Solvatochromism

When a solution changes color depending on the solvent a given solute is dissolved in, the effect is known as solvatochromism. This effect is also said to be a "response to a change in polarity of the local environment", and thus solvatochromism can be used as an indicator of solvent polarity. Solvatochromic materials are typically dyes, and are used to differentiate organic solvents.

Pleochromism

Taking a quick break from describing the types of chromism, the topic of this post is pleochromism—which, despite the name, is not actually a form of chromism. Pleochromism is the optical phenomenon that results when a materials has a different appearance/color when viewed at different angles, or different angles of polarized light. It is not the process of a color change caused by an external stimuli, but rather the inherent property of a material.

Pleochromism results from anisotropy in crystal structures. Isotropic crystals do not exhibit pleochromism. Different crystal orientations in a material absorb and transmit different wavelengths of light, which can lead to both subtle or strong differences in visual appearances. This is most commonly seen in minerals, some exhibiting merely a change in the intensity of their coloration, others shifting colors entirely.

Some known pleochromic minerals include tourmaline, pictured above, alexandrite, corundum, and apatite. Commercial applications of pleochromism are rare, but scientists can use the effect to help identify minerals and gemstones.

Color

The first thing that must be said about color is that it is, above all else, what we have defined it to be, and the definitions for what it is can vary based on perspective. Color can be defined on the visual appearance of an object, or based on properties of light. For example, some definitions might state that black is not a color in the strict sense that there is no light ray that creates black (when referring to light, black is the absence of light, whereas white is all colors of light combined). Red, on the other hand, is a term that can describe both the property of certain objects (the notebook is red) as well as a characteristic of light (typically with a wavelength of in the 600-700 nm range).

In physics, color is generally defined within the range of wavelengths that the human eye can detect, called the visible part of the electromagnetic spectrum. That portion of the spectrum is then further broken down by wavelengths, and each part then given a name. As such, usage of the term color, and various colors given name, varies. All that being said, color in general is real and connected to physical phenomena, in that photons have energy values and wavelengths, and we have given names to photons of certain energy values. This series of posts will often refer to color under the understanding that the term is defined as “a characteristic of light”, not merely the property of a physical object.

Posts for this month will start off with some reblogs of older posts about luminescence, shift into information about chromism, and end with some information about the sources of color. New posts written by this blog will have the additional tag June25Posts, relevant reblogs of this blog’s posts will be tagged June25Reblog, and everything else will have the regular relevant tags as normal. (Feel free to check out the color tag, or the structural color tag!)

Image sources: (One—rubies), (Two—pentaamminenitritocobalt(III) chloride), (Three—chlorine gas), (Four—uranium glass), (Five—LEDs), (Six—chromium chloride), (Seven—manganese chloride) (Eight—amorphous boron) (Nine-krypton)

Sources of Color: Organic Compounds

Color in many (but not all) organic materials is caused by the presence of particular organic compounds. In plants, chlorophyll is responsible for the green color most of them exhibit, while the colors of many flowers and fruits are caused by organic compounds like carotenoids, flavonoids, betalains. The same mechanism is responsible for color in most organic dyes and pigments, such as the indigo dye that is used to color jeans, as well as pigmentation in animals. Some animals get their coloration, like flamingos, from organic compounds in the food they eat. Melanin is the main pigmentation found in mammals, and can also be found in other species. Albinism is the absence of this pigment.

Sources/Further Reading: (WebExhibits) (University of Wisconsin)

Sources of Color: Dispersion

Not all sources of color arise from absorption and transmission of light with the electrons in physical objects. Some color arises from the interactions of light, including dispersion and refraction, both of which are terms used to describe changes in the wavelength of light as it interacts with physical matter. Dispersion, or dispersive refraction, is responsible for the color of rainbows, the splitting of light when it passes through a prism, and the green flash that can sometimes be seen when the sun sets. The "fire" in gemstones is also dispersion, though the color of the gemstones themselves arise from other mechanisms.

Sources/Further Reading: (University of Wisconsin) (WebExhibits) (Wikipedia)

Sources of Color: Doped semiconductors

The mechanism that creates color in doped or activated semiconductors is much the same as that in pure semiconductors—energy bands and the resulting size of the band gap determine the resulting color. However, the difference is that the color is no longer a fundamental aspect of the material itself, but a result of impurities. Light emitting diodes (LEDs) are almost always the result of doped semiconductors, with materials mixed in different quantities to get the desired colors. (Gallium, arsenic, and phosphorus are common dopants/elements.) The color of yellow and blue diamond are caused by similar impurity/dopant atoms.

Sources/Further Reading: (WebExhibits) (University of Wisconsin) (American Mineralogist) (Electronics Tutorials)