How do color-shifting inks shift colors?
The answer is in this chapter.
Governments worldwide scurry to stay ahead of counterfeiters who are quick to use the latest technology to duplicate paper currencies. Some of the security measures now used to thwart counterfeiters are security threads and special watermarks (both of which can be seen if the currency is held up against a light) and microprinting (which consists of dots too small to be picked up by a scanner). The feature that is probably the most difficult for a counterfeiter to duplicate is the variable tint that results from color-shifting inks. For example, the “20” in the lower right of the front face of a U.S. $20 bill contains color-shifting ink. If you look directly down on the number, it is red or red-yellow. If you then tilt the bill and look at it obliquely, the color shifts to green. A copy machine can duplicate color from only one perspective and therefore cannot duplicate this shift in color you see when you change your perspective.
One of the major goals of physics is to understand the nature of light. This goal has been difficult to achieve (and has not yet fully been achieved) because light is complicated. However, this complication means that light offers many opportunities for applications, ...