Identify counterfeit bills

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. 

Photo of security thread in bill

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? 

 Material and instructions:
In this project you will find out how counterfeit bills can be identified using different methods. You learn the secret behind expensive counterfeit detection pens and may decide to make your own counterfeit pen.  
Details of this project:
More details or support on this project is available for the members of Material needed for the experiments can be purchased locally or from online suppliers.