Elenco LIGHT - Manual (Page 65)

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Light, radio signals, and sound all travel through air like waves travel
through water. To help you understand how they are like waves, you can
make a cup & string telephone. This common trick requires some
household materials (not included with this kit): two large plastic or paper
cups, some non-stretchable thread or kite string, and a sharp pencil. Adult
supervision is recommended.
Take the cups and punch a tiny hole in the center of the bottom of each
with a sharp pencil (or something similar). Take a piece of string (use
between 25 and 100 feet) and thread each end through each hole. Either
knot or tape the string so it cannot go back through the hole when the string
is stretched. Now with two people, have each one take one of the cups
and spread apart until the string is tight. The key is to make the string tight,
so its best to keep the string in a straight line. Now if one of you talks into
one of the cups while the other listens, the second person should be able
to hear what the first person says.
Project 139 Cup & String Communication
How it works: When you talk into the cup, the cup
bottom vibrates back and forth from your sound
waves. The vibrations travel through the string by
pulling the string back and forth, and then make
the bottom of the second cup vibrate just like the
first cup did, producing sound waves that the
listener can hear. If the string is tight, the received
sound waves will be just like the ones sent, and
the listener hears what the talker said.
Telephones work the same way, except that
electric current replaces the string. In radio, the
changing current from a microphone is used to
encode electromagnetic waves sent through the
air, then decoded in a listening receiver.
Cups
String
Pencil
Tiny hole
Knot
String threaded
through cup bottom
Taut string