PhotoResistor type 5516 5-10K (10 ea)

These are small Photoresistors that have a resistance of 10K ohms at 10 lux

Diameter: 0.2 Inches or 5mm (Across round section)

The cadmium sulfide (CdS) or light dependent resistor (LDR) whose resistance is inversly dependent on the amount of light falling on it, is known by many names including the photo resistor, photoresistor, photoconductor, photoconductive cell, or simply the photocell.