The ‘Escences' of Light: All About Color, Part IV

Peacock feather earrings

Yes, I know that "essences" is spelled with three S's, but that is not what we are featuring today. I want to discuss the subjects of iridescence, pearlescence, opalescence, luminescence, phosphorescence and fluorescence. We touched on these topics when we discussed structural color and observed iridescence in a peacock feather and a butterfly's wings (August 14, 2019 post). Now I want to treat iridescence and its related phenomena in greater detail.

Soap bubble

Iridescence is an optical phenomenon that appears  when certain substances seem to change colors as the viewer changes the angle of observation, or the angle of light changes. You have seen it on the surface of a soap bubble. feathers, insect wings, oil slicks on wet pavement and the insides of some seashells. Often the effect is the result of structural color caused by microstructures in the material that interfere with the direction of light rays. The word iridescent comes from the Greek word iris, which means "rainbow" and the Latin word escent which means "having a tendency toward". (Learn more about the origin of the word in our April 27, 2020 post.)

Oil slick on wet asphalt

The thickness of the layers of the iridescent materials influences the pattern of light interference. Thin films like oil slick or soap bubbles are most likely to produce temporary iridescence. In an oil slick, the different colors indicate different thicknesses in the layers of oil.

Hummingbird photos by J.J.

More permanent effects are caused by many layers of structure like those in a hummingbird's feathers, the scales of a butterfly's wing, or the layers of nacre in an abalone shell. The range of colors in natural iridescence may be narrow, shifting between only 2 or 3 colors, or as wide as the whole visible spectrum. The rainbow colors require quite elaborate arrangements of micro-structure.  A few minerals show this kind of iridescence. One of the most startling places to find the effect is from the iridescent scales on the abdomens of tiny male peacock spiders from Australia. This is easily mistaken as a threat display, but it is really part of an elaborate courtship routine. And he had better do it right or the female will kill and eat him! (You can see his performance on You Tube or BBC).

We can create artificial iridescence with diffraction, which in this case refers to the spreading of light around an obstacle or forcing it through a small opening, creating an interference effect.  We see the affects of this in DVDs, CD's, smart phones with iridescent back panels, and glitter nail polish.

Mother-of-pearl inside shell

Pearlescence is an effect related to iridescence where some or all of the reflected light is white and iridescent effects produce rainbow colors. The term pearlescence is used to describe the situation when a structure within a surface causes light to be reflected back, but some or all of the light that is reflected back is white light. However, pearls can be many colors besides white — pink, blue, green, gold, grey or rainbow-colored. Artificial pearlescence can be made with certain pigments and paints. This effect is largely used in the automobile industry.  Pearlescent paints and finishes can be found in craft paints and nail polishes, as well.

Fresh water pearl jewelry

Natural pearls occur in the soft bodies, or mantles, of mollusks like oysters and some species of freshwater mussels. The pearl is composed of calcium carbonate in response to the presence of an irritating intruder in the shell. Most people believe that the irritant is a grain of sand, but it is far more likely to be a parasite. A sort of allergic reaction by the mollusk causes a pearl sac to close around the intruder.  Tiny crystals are deposited around the sac in layer after layer. This process goes on as long as the mollusk is alive. Perfect pearls are round, but pearls come in many shapes, depending upon the shape of the irritant inside. Irregularly-shaped pearls are called baroque pearls. Cultured pearls are "grown" at pearl farms where small particles are inserted into the mollusks' bodies. In this way the size and shape of the resultant pearl can be controlled.

Opalescence is an optical phenomenon displayed by the gemstone opal. This is due to the layering, spacing, and size of the many crystals of silicon dioxide, in the form of microscopic spheres, and the included water and/or air in their microscopic structures. When the sizing and spacing of the spheres is small, refracted blue-green colors are prevalent, sometimes with flecks of other colors.  This is precious opal. You can see it in the opal set into the head of this silver fish pin. The dark spot in the "eye" is a bit of stone around which the opal formed.

Precious (top) & common white opals

When the sizing and spacing of the molecules is somewhat larger, the colors yellow, orange, and red appear. This kind of gem is called fire opal. When the sizing and spacing is larger still, a milky, hazy sheen of light appears within the stone. This material is called common opal.  Exposure to heat can cause the water in the stone to break down and the fire and glow disappears. The stone is said to 'die'. During the Bubonic Plague, this effect was seen when the wearers fell prey to the extremely high fevers that the disease caused. Because people at that time did not understand diseases very well, the opals were sometimes blamed for the wearers' deaths. It became a forbidden gem and didn't regain its popularity until the 20th Century.

Honduran black opal

Opal can be found as inclusions in many different minerals. It can form in wood which later becomes petrified. It has also been found in fossil shell and bone. It is a fairly soft material, which is often "sandwiched" between a layer of dark stone, to reflect more of its brilliance, and a hard, clear quartz to protect it from wear.  It needs to be protected from heat, and opal rings or bracelets should never be worn while washing dishes.

Phosphorescent pigment before & after 'charging'

Phosphorescence occurs when the phosphorescent material is "charged" by exposure to light — usually ultraviolet light. Then the energy that has built up is stored for a period of time and released slowly.  Some minerals, including some diamonds, are naturally phosphorescent. Oddly, phosphorus, for which the phenomenon was named, is not one of them. These materials can sometimes be activated by body heat or by being vigorously rubbed. When the energy is released immediately after exposure to light, the phenomenon is called fluorescence. The study of phenomena like this led to the discovery of radioactivity in 1896.  There are practical applications, such as fluorescent lights and items like clocks that glow in the dark.

Scorpion fluoresces under UV light

Incandescent steel

Luminescence, sometimes called cold-body radiation, is a spontaneous emission of light from a substance that is not caused by heat.  It can be caused by electrical energy, chemical reaction, sub-atomic motion or stress on a crystal. This distinguishes it from incandescence, which is light being emitted by a substance that is being heated. For example, a fireplace poker left in the fire too long might begin to glow.

Luminescence has some interesting practical uses. Some examples are: lasers, light-emitting diodes (LEDs), dosimeters to detect harmful radiation, luminol to detect the presence of blood at a crime scene, and photoluminescence used in microbial diagnosis.  That last use is playing a part in the current fight against corona virus and other causes of deadly epidemics.

Fireflies (30 second exposure)

Bioluminescence is, to me, the most fascinating ‘escence’ of them all. It is light emitted from living organisms.  One summer night long ago I was riding around on dark rural roads with some college friends. Suddenly we saw an area ahead of us that was glowing and pulsing. We stopped, climbed over a fence, crossed a field and reached a tree-shaded place by a creek. On the ground were hundreds of small bodies flashing yellow, green, and a reddish light, more or less in unison. None of us had ever seen glowworms, but we realized that was what they were. Glowworms are the females of some species of fireflies (which we called "lightning bugs"). Their heads and eyes mature to adulthood, but their bodies remain in the larval state — unable to fly. We knelt down to watch. After a few minutes, we became aware of smaller pulsing lights overhead. The males had arrived. They flew intricate back-and-forth patterns before dropping down to mate with their choices of females. As mating progressed, the females went dark and the males flew away. They may have been an unnamed species, because they rarely flash red and the males are supposed to attract the females — not the other way around.  We walked in awed silence back to the car, aware that we had experienced something rare and wonderful.

Bioluminescent comb jellies

You may have recently seen film of surfers riding glowing waves at night along the California coast. The light was caused by algae in the water, disturbed by the rolling waves. It is a charming sight, but not a welcome one for scientists. The algae use up much of the oxygen in the water. If they stay around very long, this causes harm to the beds of giant kelp in the water, along with all the marine life that lives in their shelter. This can also be harmful to migrating marine animals like whales and dolphins.

Bioluminescent sea creatures

Most bioluminescent animals live in the sea. Exceptions are the fireflies, some earthworms, and a few cave species that use luminescence to attract prey. Click beetles use flashes of light when in flight to attract mates, but they also flock together on the ground and appear to use the lights as a warning signal there. Some fungi are luminous, perhaps to attract animals that will carry some of the fungal spores away with them.  Bacteria may be luminescent. Many organisms produce their own light, but others harbor luminous bacteria in various parts of their bodies.

In the ocean, particularly at great depths, many — perhaps most — animals have some luminous features. As we develop better and better instruments to send back information on sea-floor dwellers, we are seeing some wild and wonderful luminous species. Animals use luminosity for a number of reasons. An obvious one is to identify mates of the correct species and attract them.  A sudden flash of light to confuse or startle a predator comes in handy.  Being able to mimic another animal — particularly one that is poisonous or just tastes bad — is a distinct advantage.

Angler fish (public domain illustration)

A marine animal that spends a lot of time close to the surface, like a firefly squid, may have luminous organs on the underside of its body that break up its outline and allow it to appear to be part of the surface waves. The angler fish dangles a luminous lure above and in front of its huge mouth to collect unwary prey. His stomach stretches so much that he can eat animals that are nearly as big as he is!  Some sea creatures use clouds of luminescent bacteria the same way the octopus uses ink — to provide a "smokescreen" while making a quick getaway. Animals that regularly eat luminous prey usually have opaque black stomachs. After all, it doesn't pay to advertise your last meal to a predator who might make you its next one!!

I hope you have enjoyed our excursion into the ‘escences’.

 This post by Annake's Garden is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.