A self-contained unit to measure and study refraction of light. Incident and refractive rays are easily seen and measured. Light rays air into water, water to air and water in water can be studied easily with the help of built-in laser, as a light source, which revolves and can be fixed at any point around a circular graduated scale. With leveling screws to level the water in the tank. Circular scale 160 mm ( 6-3/8 in.) Diameter.