I keep an aquarium on my desk – just a little community tank with an ordinary grouping of fish: angelfish, corys, guppies and cardinal tetras. All have fascinating aspect that I enjoy watching but, from across the room the cardinal tetras, always catch my eye. Cardinal tetras are little, peaceful, schooling fish about 3 cm long, with colourless fins and two stripes running the length of their bodies; the top stripe is an iridescent blue and the bottom one is a vivid red. It's the iridescent blue stripe that grabs my attention. Like the deep greens in a rooster's tail or the rainbow colours in an oily puddle, the shimmering blue in my cardinal tetras originates from the optical phenomenon of iridescence.
Iridescence is the result of light striking a thin layer similar in width to the incoming light's wavelength, also referred to as the distance from crest to trough. Because the layer depth is close to the light's wavelength, light is reflected off both the top and bottom of the layer. As light bounces around, it interferes with itself. Interference is when two or more waves, alternately a wave and its reflected self, encounter each other and their amplitudes combine. Colour is simply light of a specific wavelength, so to amplify one colour, the two waves meet 'in phase' (aligned trough to trough and crest to crest). To negate or reduce another colour, the two waves meet 'out of phase' (aligned trough to crest). Because the wave alignment changes based on your viewpoint and/or changes to the layer itself, when you change your angle of view the colours change as well. An observer would see a colour of a certain hue – however if the angle of view to the layer is changed, by moving ones head or moving the layer as examples, the colour can appear different. This explains why patterns within the surface of an oil slick appear to swirl and shift as wind and water move the surface. Sometimes, interference can be the result of many layers of semi-transparent surfaces, where phase shifts may also occur in conjunction with many reflections and interference opportunities, creating multiple layers of iridescence.
If I took a picture of my fish, assuming they would stay still long enough for me to do so, the iridescence would not be reproduced – ordinary pigments and computer screens can't do it. Instead I would need to make a hologram to capture the effect. A hologram also works through tricks with light, this time by interference and diffraction.
Back to my iridescent fish. The pearly blue stripe results from the presence of Guanine crystals in the cardinal tetra's skin. These tiny multi-layered crystals are grown in the shape of a dinner plate within individual, almost dry chambers in the fish's skin. Somehow the fish can control the shape of the growing crystals, because when these crystals are grown in a lab, they form a much more three dimensional shape. The fish's crystals form a layer compatible with the wavelength for the colour blue, giving the fish a shiny blue stripe. So why do fish need this iridescent shininess? For some fish, the iridescence provides camouflage against the shimmering water surface when a predator is looking up from below or as a large school, little fish could produce a display so visually stunning that their predator gets confused. For my cardinal tetras, since they originate from tannin-stained, blackwater rivers in South America, most likely the iridescent strip allows them to find each other in the murky water and stick together in a school.
Someone, not me, could isolate these crystals and use them to add iridescence into lipstick, nail polishes and other cosmetic items (which is done). It takes about a ton of fish to make only 250 g of Guanine crystals. As I only have four cardinal tetras and no need of fancy lipstick or nail polish, I'll continue to enjoy just watching my fish swim.
thanks to G. Hanke for the photo as I didn't have the patience to wait long enough to get one.
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