How Does It Work?: Holographic Nail Polish
Unlike traditional nail polishes, which get their color from various pigments, the colors in holographic nail polishes are derived from structural factors.
We call these polishes “holos”, but, in fact, they are displaying a characteristic traditionally referred to as iridescence. Iridescence is defined as “a change in the hue of the object exhibiting it as the angle of vision is varied”. In Greek mythology, the god “Iris” was the personification of the rainbow. Thus, the term “iridescence” comes from the Greek word for rainbow. This rainbow is clearly evident in holographic nail polish.
|Color Club’s “Harp On It”|
The mechanism of this rainbow pattern is based on simple properties of physics.
Before we get into them, though, we are going to need to talk a bit about light.
Light if a form of electromagnetic radiation. It is emitted in elementary particles known as photons. It displays characteristics of both waves and particles. Lots of kinds of waves exist in the electromagnetic spectrum. However, we only can see a small amount of those waves. Whether or not we can see light depends on the light’s wavelength. A wavelength is the distance before the wave’s shape repeats. The magnitude is how high the wave goes (on the graph). A higher magnitude wave would be perceived as brighter.
Visible light is light that falls between 380 and 740 nanometers. (For reference, there are 1,000,000,000 nanometers in a meter.) A slightly longer wavelength produces infrared radiation, whereas a slightly shorter wavelength produces ultraviolet radiation. Different wavelengths within the visible spectrum create different colors of light. White light contains all wavelengths of visible light.
If an object absorbs all wavelengths of light, it is perceived as black. If it absorbs certain wavelengths but not others, it is perceived as the colors of the wavelengths that are reflected back at you. (For example, chlorophyll, the photosynthetic pigment that famously gives plants their green color, is capable of utilizing all wavelengths except green. The green is reflected back at you, and thus chlorophyll is perceived as green.) If an object reflects all wavelengths of light, it is perceived as white.
Thus, you can probably predict how holographic nail polishes get their effect: by utilizing structural tools, they are able to change how light reflects and thus get a unique pattern. Indeed, the rainbow-like hues that occur in holographic nail polish are
caused by two factors working together: interference and diffraction. Both of these processes fall under the broader umbrella of “coherent scattering”, in which ordered light scattering elements produce distinct patterns.
Diffraction is a process that occurs when a light wave encounters an obstacle. In essence, the light wave ‘bends’ around the obstacle.
|The light seems to ‘bend’ around the corner in this picture.|
Diffraction grating consists of a series of grooves over a reflective surface. Some of the light is reflected normally, whereas the rest is split into its component wavelengths and reflected in different directions.
Because light with a longer wavelength has a higher diffraction angle than light with shorter wavelengths, the light separates into its component parts, creating distinct bands of color in the order of the rainbow.
|Diffraction creates these rainbow bands of color.|
In holographic nail polish, the physical properties required for diffraction grating are present in the silver particles in the nail polish.
Interference is also an important component of holographic nail polish patterning.
Although light has properties of both waves and particles, for the purposes of this explanation, we are going to focus on light’s wavelike features.
Waves of light that are the same wavelength can interact in two ways: via constructive interference and destructive interference.
|Constructive and destructive interference|
Constructive interference occurs when the two waves of light are “in phase”. That means that the phase difference is in multiples of 2π. The phase difference is the distance between the two waves having the same frequency. Because the wave is a simple sine graph, it repeats its pattern every 2π. That means that the two wavelengths look essentially the same. In constructive interference, the two waves interact to create a resultant wave that has twice the magnitude of the original waves. When a viewer sees visual light that has been enhanced by constructive interference, they see a very bright light.
Destructive interference occurs when two waves of light are “out of phase”. That means that the phase difference is in π, 3π, 5π or so forth. In this case, the two waves cancel each other out. When a viewer sees visual light that has undergone destructive interference, they do not see any light at all.
If they are not in multiple of pi, you get a combination effect, creating a new wavelength from the two old wavelengths.
So, how does this contribute to the “holographic” effect in your nail polish?
The silver particles in your nail polish are coated with a thin layer. When the light hits the silver particles in your holographic nail polish, some of it bounces off the top layer, whereas some of it penetrates and then bounces off the particles themselves. When it enters the film, its wavelength changes. These waves of light can interfere with each other, forming a brand new wavelength that is slightly out of phase.
|Thin film interference, illustrated.|
Both the angle of incidence and the thickness of the film can play a role in how these two wavelengths interact. Thus, when you change the angle you are using to examine your beautiful nails, you are changing both of those factors (film thickness changes because, depending on the angle, the light may have further to go or less far to go to reach your eye). As a result, the color seems to change as you move around. At certain angles, there will be constructive interference, whereas at others there will be destructive interference. As certain waves are canceled out (via destructive interference), a pattern emerges.
A tapered film causes the wavelengths to differ at different places. Consequentially, you will end up with a series of parallel fringes that also create a rainbow-like pattern, just like you might see in a soap bubble, since certain parts of the bubble are thicker than others.
When diffraction and interference work together, they can create startling patterns in your nail polish known as holographic effects.