Monterey Bay Aquarium Research Institute
Marine Botany

Mazzaella flaccida (was Iridaea): The Incidental Beauty

One of the most distinctive traits of I. flaccida (and Chondracanthus, a subtidal Gigartinale) is the iridescent sheen of its thallus. Multiple colors seem to swim on the surface of this algae, much as do the colors on the surface of soap bubbles. The many thin layers of Iridaea's surface cuticle reflect and refract light, producing iridescence by means of the constructive and destructive interference of light. 9 This effect is especially pronounced subtidally where a snorkeler may see liquid patterns of color moving with the waves above brown, encrusted rock faces. The color of light reflected by the thallus of these algae changes with the angle of incident light, which itself changes as the alga moves with the waves. The physical laws of light help us to understand how iridescence is produced and why the palette of colors changes with the angle the thallus is observed.

(Unfortunately I have run out of time and I cannot include anything more than my notes from my presentation on the fascinating topic of iridescence. The above introductory paragraph outlines the topics I would like to include. Following are all parts of my presentation which make sense in the absence of further explanation.)

Is iridescence a material property?

Not directly. Iridescence is a consequence of the arrangement of material (thin layers) rather than its nature. Of course, the material must be at least somewhat transparent to light; the phone book might be made up of thin sheets, but it certainly isn't iridescent. The composition of the cuticle is quite ordinary9:

protein 50%

carbohydrate 40%

inorganic salts 5%

fatty acids <1%

The cuticle does not affect rates of photosynthesis or respiration. Why bother having one?

Gerwick and Lang suggest it may help prevent desiccation and deter hungry herbivores. 8 In trouser tear tests, the cuticle rips in such a way as to increase the force necessary for propagation of a crack. Apparently the cuticle helps to prevent the thallus from tearing. More details to come....

Why is Iridaea more iridescent when wet than dry?

The outer surface of the cuticle is composed of flake-like material. Water smoothes the flakes and allows a more uniform reflection of light rather than random scattering.

Why is Iridaea more iridescent than a soap bubble?

Iridaea's cuticle is multilayered (up to 17 layers!) and each of these layers has a slightly different thickness. The wavelength of reflected light is a function of the thickness of the layer. The greater number of layers of slightly differing thickness in Iridaea allows more refraction and interference, and thus more intense iridescence. A soap bubble has only one layer, although its thickness also varies, producing a variety of hues.

Why do you see so many colors on the thallus?

The cuticle layers do not have a uniform thickness, so constructive interference occurs in some parts while destructive interference occurs in others, producing different colors.

Why these colors?

Sorry, this takes a bit more physics. The color depends on the wavelength of light after interference has occurred. The wavelength depends on the index of refraction (how much the light was bent when it entered and exited the cuticle), the angle of incidence, and the original wavelength of the light.

The index of refraction comes from Snell's Law:

index of refraction

= sin i / sin r

= u1 / u2

= speed of light in initial medium) / speed of light in final medium

= n

where i, r = angle of incidence, angle of refraction.

The color you see depends on the light's wavelength when it reaches your eye. Several fairly intuitive formulas help you figure out this wavelength.

Some variables:

|\ = wavelength

d = thickness of layer

m = constant = (0, 1, 2...)

(you have interference regularly every constant interval because light is composed of waves)

Minimum intensity

(destructive interference causes a minimum wavelength)

2 d n = m |\

(The light refracted inside the thin film must travel a distance of 2d if it enters normal to the surface. If |\ (wavelength) is any constant multiple of this distance, light returning from the back surface of the film will be in phase and interfere with light reflected simply from the top surface of the film.)

Maximum intensity

(constructive interference gives a maximum wavelength)

2 d n = (m + 1/2) |\

(The term of 1/2 is introduced because the ray reflected from the top surface experiences a phase change of when the medium beyond has a higher refractive index. Cuticle certainly bends light more than water, so has a higher refractive index. To understand the need for the 1/2 intuitively, tie two strings of different thickness together and send a wave down them.

-> <- ->-------/\----===== => --------\/---===/\===

The wave both reflects and passes through the thickness interface, but the reflected wave is now shifted in phase by and returns as a depression in the string rather than a lump.)

Complicating Factors:

Bubble Thickness

When the thickness of the bubble is not an exact multiple of the light's wavelength, only partial interference occurs. The light reflected off the surface and the light reflected from the "back" side of the thin soap film will not be exactly out of phase, so these light waves will not completely interfere with each other. Otherwise, we would not see the object at all, let alone its colors!

You can see the influence of thickness by observing a soap bubble as it begins to drip; as bubble material decreases, its walls thin and its colors change.

Angle of Incidence

In real life, incident light will not strike the surface at a perfect 90 degree angle. Since most light actually does not strike the surface dead on, waves of light from any real source will experience different amounts of interference. A real source will have to emit light in something less than a perfect beam, so incident waves will not hit the surface in precisely the same location. Discrete light waves which encounter a curved surface (such as a soap bubble) will experience different bubble thicknesses because they are not incident on exactly the same location. Moreover, real material, such as a bubble, does not have a perfectly uniform thickness, so these waves will experience different refraction and reflection pathways.

This phenomenon is easy to demonstrate; just change the angle from which you are examining your soap bubble, and the colors on its surface will change. Similarly, different colors will appear to wash across the surface of Iridaea if you bend down and move your face closer to the level of its holdfast.

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