What you need:
• A clear water bottle
• Duct Tape or aluminum foil and tape
• Thumb tack or push pin
• Sink or basin
What you do:
• Remove the label from the water bottle.
• Cover half (vertically – from opening to bottom of bottle) of the water bottle using duct tape or taping on aluminum foil.
• Use the thumb tack to poke a hole through the duct tape on the bottle, about two inches from the bottom of the bottle.
• Use a pencil or similar object to enlarge the hole.
• Fill the bottle with water, keeping your thumb over the hole so the water stays in.
• In a very dark room, hold the water bottle over the sink or basin.
• Shine a flashlight through the uncovered side of the bottle toward the hole where the water is coming out.
• Watch as the stream of water forms an arc that the light follows down into the sink.
• Try making the water stream a different color by putting colored cellophane in front of the flashlight lens
N.B.: The light beam travels through the stream of water, even when the stream bends, as it does in this experiment. The light beam bounces off the walls of the water stream and follows it to the end. This is called internal reflection. The light ray inside the stream of water behaves as it would inside an optical fiber. Optical fiber works like this: you send a light beam into one end of the fiber and it comes out the other end, even when it bends, just as light travels through the stream of water in your experiment.