Waves can be divided into two types - called transverse and longitudinal - depending on how they travel.
A transverse wave travels like this.
To make a transverse wave, take a rope, tie it to something at one end and wiggle the other end up and down. You will see a wave move
along the rope away from you.
A particular bit
of the rope will move up and down, but will
not move ‘along’ the rope. The
direction in which the wave moves (along the
rope) is at right angles (perpendicular) to
the direction in which the rope moves as the
Waves on the surface of water (for example in a pond) are also an example of transverse waves.
If you watch a boat on the sea, the boat just bobs up and down on the water as a wave moves past it.
Sound waves are longitudinal waves. As the wave moves, the molecules in the air move backwards and forwards in the same direction as the wave.
To make a longitudinal wave, you'll need something "squashy".
Take a slinky (spring), rest it on a polished
(shiny) table and fix one end of it. Hold the
other end with your hand and give it a sharp
push in the direction of the spring. The
part of the spring nearest your hand will squash
(compress) when you push it and that compression
will travel along the spring.
compression is a longitudinal wave.
The displacement of the medium (spring)
is in the same direction as that in which the