Introduction: Whether altering the design of ancient coins or making photocopies of bills with high-tech machines, counterfeiters have made illegal money throughout history. Merchants on the other hand try to protect themselves by testing and identify counterfeit bills in any possible way.

I have seen two different methods of identifying counterfeit bills. In a bank I noticed that the cashier holds the bill under a purple light. In a supermarket I saw the cashier puts a check mark on the bill using a special marker or pen. 

But, how do these methods work? How can a bank clerk pass the bill under a purple light and quickly determine if it is a valid bill. What does the marker do?