Lesson 2 of 10 · 7 min
Transverse and longitudinal waves
NCERT §14.2
Mamaji ties one end of a long string of beads to a hook and gives the other end a single flick. A hump runs along it. Then he pushes the handle of a hand pump in and out, and at the far end of its hose a scrap of paper flutters. Two waves, moving in two quite different ways.
The lesson in notes
In short
Give one end of a stretched string a single up-and-down jerk and a pulse runs along it. A continuous periodic jerk sends a train of pulses: a sinusoidal wave if the end moves in SHM.
In a transverse wave the particles of the medium oscillate at right angles to the direction the wave travels, as on the string.
In a longitudinal wave the particles oscillate back and forth along the direction of travel. A piston pushed and pulled at one end of an air-filled pipe sends out regions of high density, compressions, alternating with regions of low density, rarefactions. Sound in air is longitudinal.
Both are progressive (travelling) waves: the pattern moves from one part of the medium to another.
A transverse wave needs a medium that resists a change of shape (shear), so transverse mechanical waves travel only in solids and along strings, not through the body of a liquid or gas.
A longitudinal wave needs only resistance to compression, which solids, liquids and gases all have. A steel bar can carry both kinds; air carries only longitudinal waves.
Surface waves on water come in two types. Capillary waves are ripples no more than a few centimetres long, held by surface tension. Gravity waves have wavelengths from several metres to several hundred metres, and gravity supplies their restoring force.
In water waves the particles move both up and down and back and forth, and the motion reaches down to the bottom with falling amplitude. Ocean waves combine transverse and longitudinal motion.
In the same medium, transverse and longitudinal waves generally travel at different speeds.