How were variable displays made bright and counterfeit-proof?
The answer is in this chapter.
One security feature on credit cards, driver’s licenses, and many other types of identification cards is a variable display that changes contents when a card is tilted. Such a variable display is often called a hologram, which is a type of photograph that gives depth and perspective to its images. Indeed, the earliest versions of these variable displays on credit cards were actual holograms. However, the use of holograms as a security measure was not very successful for two reasons. First, they were dim, murky, and difficult to see in the lighting of a typical store. Second and far worse, they were easily duplicated by counterfeiters. Today the variable displays are bright, sharp, and easily seen in store lighting. Better, they are extremely difficult to counterfeit.
One focus of physics in the study of light is to understand and put to use the diffraction of light as it passes through a narrow slit or (as we shall discuss) past either a narrow obstacle or an edge. We touched on this phenomenon in Chapter 35 when we looked at how light flared—diffracted—through the slits in Young’s experiment. Diffraction through a given slit is more complicated than simple flaring, ...