These cadmium-sulphide (CdS) photocells are effectively resistors that change value of resistance with ambient light level. The more light strikes the sensor face, the less electrical resistance the sensor offers.
These optical sensors work very well for detecting model trains when placed between the rails due to the shadow that a train casts. Ambient room light is required for these sensors to function correctly.
One package contained 10 CdS photocells.