Applying Fluorescence to Minimize Counterfeit Money  

 

To make counterfeitting more difficult many governments, including the U.S., are using fluorescence. You can see this effect by exposing any new style U.S. currency to UV light. Near one end, a line will appear that cannot be seen with visible light. This fluorescent thread is visible on the front and back of the bill.

 

Use your knowledge of fluorescence to describe the process by which this part of the currency emits visible light. Then, describe the nature of the solid that is needed to create an appropriate band structure for this process.