Types of Mechanical Waves:

Depending upon the direction of displacement of medium with respect to the direction of the propagation of wave itself, mechanical waves may be classified as longitudinal or transverse. Longitudinal waves can be produced on a spring (slinky) placed on a smooth floor or a long bench. Fix one end of the slinky with a rigid support and hold the other end into your hand. Now give it a regular push and pull quickly in the direction of its length. A series of disturbances in the form of waves will start moving along the length of the slinky. Such a wave consists of regions called compressions, where the loops of the spring are close together, alternating with regions called rarefactions (expansions), where the loops are spaced apart. In the regions of compression, particles of the medium are closer together while in the regions of rarefaction, particles of the medium are spaced apart. The distance between two consecutive compressions is called wavelength. The compressions and rarefactions move back and forth along the direction of motion of the wave. Such a wave is called longitudinal wave and is defined as:

                             In longitudinal waves the particles of the medium move back and forth along the direction of propagation of wave. 

We can produce transverse waves with the help of a slinky. Stretch out a slinky along a smooth floor with one end fixed. Grasp the other end of the slinky and move it up and down quickly (Fig.10.9). A wave in the form of alternate crests and troughs will start travelling towards the fixed end. The crests are the highest points while the troughs are the lowest points of the particles of the medium from the mean position. The distance between two consecutive crests or troughs is called wavelength. The crests and troughs move perpendicular to the direction of the wave. Therefore, transverse waves can be defined as: 

In case of transverse waves, the vibratory motion of particles of the medium is perpendicular to the direction of propagation of waves. Waves on the surface of water and light waves are examples of transverse waves.