Before explaining how opal’s play-of-color works, I’ll need to review a few definitions you may not have encountered since school.
Definitions
Visible Light
Visible light is electromagnetic radiation that the human eye can see. It forms as a wave with wavelengths between approximately 380-740 nanometers (nm).
White Light
White light is a combination of light of multiple wavelengths. It’s generated by the sun, stars, fluorescent lamps, white LEDs, and incandescent bulbs
Wavelenghts of Visible Light
The University of Waikato Te Whare Wānanga o Waikato
Parts of a Light Wave
The parts of a light wave include crest, trough, amplitude, and wavelength. The crest is the peak, the trough is the lowest part, the amplitude is the distance from the midline to the top of a peak or the bottom of a trough, and wavelength is the distance from one crest or trough to the next.
Parts of a Wave
Destiny E.
When Light Encounters a Barrier
When a wave encounters a barrier, it behaves in one of three ways: reflection, refraction, or diffraction.
Reflection
Reflection occurs when a wave traveling in a straight line bounces off a barrier. The angle at which the wave approaches the barrier always equals the angle it reflects off it.
The Law of Reflection
the Physics Classroom
Refraction
Refraction occurs when a wave changes speed as it crosses from one medium to another, redirecting the wave. At any change in medium, like when light traveling through a prism becomes light traveling through the air, shorter wavelengths will deviate more from their original path than longer ones. As a result, the white light breaks up or disperses into its component colors as it exits the medium.
Light Dispersion through a Prism
Edmund Optics
Diffraction
Diffraction occurs when a wave bends around a barrier or through an opening smaller than the wavelength. Because the medium on both sides is the same, the speed and wavelength don’t change, only the direction in which the wave travels. Smaller gaps will change the direction more than larger gaps. If the gap is too large, the wave will just pass through it with no change.
When a wave encounters multiple gaps close together, the resulting changes in direction will interfere with each other in constructive and destructive ways. When a wave at its crest meets a wave at its trough, they will cancel each other out. If a wave at its crest meets another wave at its crest (or a wave at its trough meets another wave at its trough), then it will increase the amplitude and create a super crest (or super trough). If a wave in the positive half of its waveform meets a wave in the negative half of its waveform, it will destructively interfere and produce a wave whose amplitude is less than either of the original waves. Such a decrease in amplitude will reduce the intensity of the light, while a super crest or trough will increase the intensity of the light. This results in a pattern of bright and dark stripes.
Diffraction of Light Through Wide and Narrow Openings
Alternative Physics
Constructive & Destructive Interference
Alan Zucchoni
With white light, the shorter wavelengths will diffract at greater angles than the longer wavelengths, splitting the light into its component parts.
Dark, Light, Dark Light Pattern
Michael Brush
Constructive Multiple Split Diffraction
Lumen Learning
Diffraction Grating
A diffraction grating is the name given to a repeating structure that diffracts light. There are two types: transmission and reflection. A transmission grating is similar to the images above, with holes at a regular spacing dispersing light waves.
A reflection grating has ridges and reflective surfaces that reflect the light, causing the same light wave dispersion.
Reflection vs Transmission Diffraction Grating
Wastach Photonics
Opal's Play-of-Color
In precious opals, the silica spheres settle in a regular and closely packed lattice that mimics crystal structure.
I assumed the spaces between the silica spheres caused the opal lattice to act as a transmission grating, but this isn’t what is happening. Instead, some light reflects off the top-level silica spheres, while some refracts through the silica spheres to lower-level spheres, reflecting or refracting again.
The silica spheres act like a reflection diffraction grating and disperse the light into its components instead of behaving like typical light reflection. Then all of the reflected light waves interfere, causing some wavelengths within the visible light spectrum to amplify and other wavelengths to cancel out
Refraction and Diffraction of an Opal
Geoffrey A. Ozin, Andre C. Arsenault
When you rotate the opal, the angle at which the light enters the opal
changes, changing the angle of reflection. This changes where the interference occurs, making it look like the color is flashing.
The size of the spheres controls color. The spheres must be between 150-350 nm, roughly half the wavelength of light. Too large or too small, the light won’t break into its component parts. At 150 nm, the sphere will only bend light at the blue end of the spectrum. At 350 nm, the sphere will bend any color of visible light. When you rotate the opal and change the angle of the light, the same spot may appear to be a different color.
In Australian opals, small silica spheres are typical. Most are blue, so red is the most prized color. Ethiopian opals more often show red and green. This means Ethiopian opals have larger spheres and are more colorful than Australian opals.
Thin-Film Interference
The colors in minerals like labradorite happen because of a similar process, thin-film interference.
When you have two thin boundaries that light passes through, part of it will reflect normally, and part will reflect through the top boundary and reflect off the bottom. The reflected waves will then interfere with each other. Because there is less dispersion of color, the bandwidths are narrower. Full rainbow-color wavelengths rarely happen. Instead, you see more pinks, teals, and golds.
References
This article took hours of discussion and numerous emails back and forth with the late Dr. Don Hoover, FGA, FGAA (Hon.), before I could completely grasp the concept of the reflection diffraction grating. I am eternally grateful for his mentorship and friendship.
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https://sites.google.com/a/wcsga.net/vanessa-rodriguez-a3-physics/unit-10-pt-1-1 (Accessed 3 Jul 2023) - [secrop] (2014) A complex approach – Iridescence in cycles. [Blender Artists] Available at: https://blenderartists.org/t/a-complex-approach-iridescence-in-cycles/613211/38 (Accessed 3 Jul 2023)
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Last Updated on 8 December 2023 by Angel Doran