ELi5: Why does colour shifting occur on Canadian banknotes


I’m doing a research project for a chemistry class and we need to create a brochure about how chemistry is used to battle counterfeit currency.
Chemistry is not my subject at all.
While I have most of the project done, can someone explain why, on Canadian banknotes, colour-shifting occurs? I’m referring to aspects like the feather on one bill going gold to green when moved up and down.
Thank you!

In: 2


This is called optically variable ink.
It changes the reflected wavelength of light when viewed at different angles. It’s paint with suspended particles that reflect differently at the viewing angles.

Part of it is chemistry though I think a good part of it is also physics, OVI’s are multiple layers of ink with particles that effectively bounce the light different. There are filter layers in between to cancel out/or change the different colors that appear. It’s not a perfect example but this [illusion with the transparent](https://www.youtube.com/watch?v=lvvcRdwNhGM) filtered lines changing how the background image is perceived I think is a close explanation. It’s not perfect, but kind of close.
I suppose the mixtures of the paint are chemistry to get the right layers but depending the level of the course I suppose an argument about using suspended particles and filter layers to change light could overlap with physics as well.

There are some versions that go on in one layer, though the specific processes for official currency are a little more exact.

Your question is ambiguous, and could be one of two questions:

1. Why use color shifting ink?

It’s used for forgery detection. It isn’t easy to design and print an ink that does this–making forgery more difficult and expensive to do. Also, you cannot just photo copy the bill as it won’t exhibit the color change–it’ll be one color or the other.

2) How does color shifting ink work?

It requires that the color seen depends upon the angle of incidence. Instead of using pigment molecules like a normal paint, color changing ink uses what is called [structural coloration.](https://en.wikipedia.org/wiki/Structural_coloration) That is, the color is at least partially generated due to the geometry of surface and not just on what colors the material absorbs.

There are numerous ways to do this, but one way is to make use of an optical resonator. Place two reflectors a certain distance apart. The top reflector must be partially transparent to allow light to enter depart. The two reflectors are arranged such that the distance between them is the same as the wavelength you want to display. Due to standing waves the optical resonator will absorb that light which is does not have a wavelength equal to the separation distance (this is the same reason a bottle makes a specific note when you blow across it–we’re just using light instead of sound). Now if you look at this resonator from an angle off center, then the light is now traveling at a diagonal through the resonator. Because it is going at a diagonal, the distance between the reflectors is effectively farther apart (since the light ray has to travel farther between both reflector just like the hypotenuse is longer than either of the other sides) which shift the wavelength lower–making a different color. This is the method butterfly wings use to make their iridescent colors. It was also used in Mirasol display technology for color e-ink.