How do holographic nail polishes work?
White light is actually made up of many colours, and can be split - such as in rainbows. Holographic particles do this too - they're silver particles with a coating which splits or diffracts the light into a colour spectrum:
(Note: this is a very simplified diagram. The production of the spectrum is actually a bit more complex and involves patterns of constructive and destructive interference, but the idea is essentially the same.)
The main thing which changes how "linear" a holo polish will be is how smooth the layer of holo particles is:
Generally, the smaller the particle size and the denser the polish, the smoother the holo layer. If the layer is smooth, then the colours reflected go gradually through the colour spectrum and so you get a linear holo effect. But if the layer is less smooth, then the colour change is less gradual and you end up with a scattered effect. If you have a very smooth holographic surface, such as on the back of a CD or DVD, you will get a perfect uninterrupted rainbow.
I swatched a Spectraflair topcoat over a black creme to show the effect:
From left to right:
1. Incomplete coat - moderately scattered
2. One coat - mostly linear holo
3. Two coats - similar to 2 but more silver due to greater reflectance from more particles
4. Three coats - less linear than the other coats, despite more holo particles - repeated brushing has made the top layer uneven. Very silver.
So application and polish formula is important in determining how scattered a holo is as well.
Hope you enjoyed that explanation!