Turn on the underwater light and observe the angles of refraction of various light beams as they break through the water's surface into the air - a medium with reduced optical density. Angles vary all the way from zero to full reflection, when the beam just breaks at the water's surface and return right back into the water.
Scientific Principle: Light refraction
Needed Infrastructure: Electricity and darkness
Size in mm: height: 300
depth / length: 450