Young's experiment and the dual nature of light
The double slit experiment proves the wave properties of light.
Vary the size of the slits. Do the patterns on the screen change?
Put pin holes into the cardboards. Make holes in different locations. How do the patterns on the screen change?
Double slit experiment - http://en.wikipedia.org/wiki/Double-slit_experiment
Photon - http://en.wikipedia.org/wiki/Photon
Wave-particle duality - http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
Basic safety requirements.