ca$h money physics

have you ever tried forging currency? i know i have. turns out, its not as easy as it looks. paper money has many copy-resistant properties that people can use to distinguish real from fake money. one example of this is the ink used to print a portion of the bill. this ink is special because it changes color depending on the angle it is looked at. this is achieved by infusing the ink with microscopic prisms that have sides that reflect different wavelengths of light. they also have strips of paper inside of them with ink that glows under ultraviolet light. like the highlighters i discussed in a previous post, these strips have a special chemical that turns the ultraviolet light into visible light, a trait that is very useful for detecting fake money.