Types of Waves in Physics – Definition, Types – eSaral

Class 9-10, JEE & NEET

Hey, do you want to learn about the types of Waves in Physics? If so. Then keep reading.

## Waves:

A wave is a disturbance that propagates energy from one place to the other without the transport of matter. It is an ill spread over a region of space without clear-cut boundaries. It cannot be said to be localized here or there.

For the propagation of a wave, there are two properties in a medium.
1. Elasticity
2. Inertia

## Types of Waves:

### Mechanical and Electromagnetic waves

A wave may or may not require a medium for its propagation. The waves which do not require a medium for their propagation are called non-mechanical, e.g., light, heat, and radio waves are non–mechanical as they can propagate through a vacuum. In fact, all electromagnetic waves (EMW) such as g-rays, X-rays, or microwaves are non-mechanical. On the other hand, the waves which require a medium for their propagation are called mechanical waves. In the propagation of mechanical waves elasticity and density of medium plays an important role. This is why mechanical waves sometimes are also referred to as elastic waves. Waves on string and springs, seismic waves, or sound waves are familiar examples of mechanical waves. One cannot listen to his companion on the moon or sound from the sun does not earth because there is no medium for its propagation.

Note: Apart from mechanical (elastic) and non-mechanical (electro-magnetic) waves there is also another kind of wave called ‘matterwaves’. These represent the wavelike properties of particles and are governed by laws of quantum physics.

## Transverse and Longitudinal waves:

### Transverse Waves:

1. If the particle of the medium vibrates at a right angle to the direction of wave motion of energy propagation the wave is called a transverse wave. These are propagated as crests and throughs. Waves on strings are always transverse.

### Longitudinal Waves:

If the particle of the medium vibrates in the direction of wave motion, the wave is called longitudinal. These are propagated as compressions and rarefaction and also known as pressure or compressional waves. Waves on spring or sound waves in the air are examples of longitudinal waves.

The transverse or longitudinal nature of a wave is decided by ‘polarization’ as the transverse wave can be polarized while longitudinal wave cannot be. This is shown diagrammatically in Fig.

## Mechanical waves in a different medium:

A mechanical wave will be transverse or longitudinal depends on the nature of the medium and mode of excitation.

In strings, mechanical waves are always transverse that too when the string is under tension. In gases and liquids mechanical waves are always longitudinal, e.g., sound waves in air or water. This is because fluids cannot sustain shear.

In solids, mechanical waves (maybe sound) can be either transverse or longitudinal depending on the mode of excitation. The speeds of the two waves in the same solid is different. (longitudinal waves travel faster than transverse waves). e.g., if we struck a rod at an angle as shown in Fig. (A) The waves in the rod will be transverse while if the rod is struck at the side as shown in Fig. (B) or is rubbed with a cloth the waves in the rod will be Longitudinal. In the case of vibrating tuning fork waves in the prongs are transverse while in the stem are longitudinal. Furthermore, in case of seismic waves produced by earth-quakes both $S$ (shear) and $\mathrm{P}$ (pressure) waves are produced simultaneously, which travel through the rock in the crust at different speeds $[\mathrm{Vs} \cong 5$ $\mathrm{km} / \mathrm{s}$ while $\mathrm{Vp} \cong 9 \mathrm{~km} / \mathrm{s}$ ] S-waves are transverse while P-waves longitudinal.

Some waves in nature are neither transverse nor longitudinal but a combination of the two. These waves are called ‘ripple’ and waves on the surface of a liquid are of this type. IN these waves particles of the medium vibrate up and down and back and forth simultaneously describing ellipses in a vertical plane [Fig.]

Explain why –
1. Transverse mechanical waves cannot be propagated in liquid and gases while To transmit a transverse mechanical wave medium must be elastic so as to provide a restoring force when acted on by shearing stress. But liquids and gases flow when acted on by shearing stress, i.e., they cannot sustain shear stress to provide restoring force and so cannot transmit transverse mechanical waves.
2. Waves on the string are always transverse. Longitudinal waves are pressure waves, i.e., they are transmitted as compression and rarefaction in a medium. Now as the string is non-stretchable so can neither be compressed nor stretched, i.e., in it compression and rarefection cannot be produced. This in turn implies that longitudinal waves can not be propagated along a string So the waves in a string are always transverse that too when the string is under tension. If the tension in the string is zero, the transverse mechanical wave will also not propagate as then $v=\sqrt{(T / m)}=0]$

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