The QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 0.5" x 0.3" module that can be mounted almost anywhere and is great for edge detection and line following applications. This sensor is sold in packs of two units.
This 5mm Water-clear Phototransistor will pick up visible and infrared light and conduct current. When light reaches this high sensitive/fast response time component it's much like current reaching the base of a transistor which allows current to flow from collector to emitter.
If you want to experiment with radio frequency identification (RFID), this book by Tom Igoe (renowned for his work with Arduino) is the perfect place to start. You’ll be guided through three hands-on projects that let you experience RFID in action.