The IED Photo Cell Switch is what you're looking for when building a training aid that goes off when light hits it.
Many of these switches look alike but they don't act alike. If you're building a trainer, this is the one you need.
Light dependent Resistor. Cadmium Sulfide type. Changes resistance with light. Max. Dark Resistance 2M ohm Resistance @ 2 ft. candle 5k ohm