A wave is a disturbance that travels or propagates and transports energy and momentum without the transport of matter. The ripples on a pond, the sound we hear, visible light, radio and TV signals are a few examples of waves.
Sound, light and radio waves provide us with an effective means of transmitting and receiving energy and information.
Waves are of two types:
- Mechanical waves require a material medium for their propagation. Elasticity and density of the medium play an important role in the propagation of mechanical waves.
- Electromagnetic waves require absolutely no material medium for their propagation. They can travel through the vacuum. Light, TV signals, radio waves, X-rays, etc. are examples of non-mechanical waves.
Transverse Waves and Longitudinal Waves
In transverse waves, the constituents of the medium oscillate perpendicular to the direction of wave propagation and in longitudinal waves, they oscillate along the direction of wave propagation.
Consider the wave traveling along a rope. The direction of propagation of the waves is along the rope, but the individual particles of the rope vibrate up and down. The electromagnetic wave (light, radio waves, etc) through not mechanical, are said to be transverse, as the electric and magnetic field vibrates in a direction perpendicular to the direction of propagation.
In these waves, the direction of vibration of the particles of the medium is parallel to the direction of propagation.
The figure shows a long and elastic spring. When we repeatedly push and pull on end of the spring, the compression and rarefaction of the spring travel along the spring. A particle on the string moves back and forth, parallel and anti-parallel to the direction of the wave velocity.
- Sound waves in air are also longitudinal.
- Some waves (example, ripples on the surface of a pond) are neither transverse nor longitudinal but a combination of the two.
- In strings, mechanical waves are always transverse, when the string is under a tension.
- In gases and liquids, mechanical waves are always longitudinal, e.g., sound waves in air or water. They possess volume elasticity, because of which the variations of pressure (i.e., compression and rarefaction) can travel through them. For this reason, the longitudinal waves are also called pressure waves.
The waves on the surface of the water are of two kinds:
- Capillary waves
- Gravity waves
Capillary waves are ripples of fairly short wavelength – no more than a few centimeters. The restoring force that produces these waves is the surface tension of water.
Gravity waves have a wavelength of several meters and restoring force is the pull of gravity.
A wave, transverse or longitudinal, is said to be traveling or progressive if it travels from one point of the medium to another.