To demonstrate the principle of refraction of light rays in water.
Take a glass tumbler of water. Place a small piece of colored paper on a table and place the glass tumbler over it. Then cover the glass with a saucer. The colored paper vanishes, and is invisible even when seen from any angle. This is due to the phenomenon of refraction, the bending of light rays when they pass at an angle from one medium to another. The lines show the path of light rays are refracted upward as they pass from water to air and how the are blocked by the saucer, which stops all refracted rays, thus the colored paper can't be seen from any angle.