Wave Motion

What is Wave Motion?

Propagation of a disturbance ( a wave ) which travel through a medium due to vibrations or oscillations of medium particles about their mean position is called a wave motion.

Wave motions have following characteristics –

Wave travels through a medium due to vibrations of medium particles about their mean position.
Energy required for vibrations is transferred from one particle to adjacent particle without actual transfer of mass.
Each particle receives disturbance ( i.e. wave motion ) a little later than its previous particle. Therefore, there is a phase difference between one particle and the next one.
The velocity of wave motion is different than the velocity of the vibrations of medium particles.
In a medium, velocity of wave motion remains constant but the velocity of medium particles varies sinusoidal.
Wave motion requires a medium to travel which possesses the mechanical properties of inertia, elasticity and minimum friction between its particles.

Types of Wave Motion

Wave motion is broadly classified into two categories depending on the relationship between the direction of wave motion and direction of vibration of medium particles. These are –

Transverse wave.
Longitudinal wave.

Transverse Wave Motion

In transverse wave, medium particles vibrate perpendicular to the direction of propagation of waves.

In transverse waves –

Displacement of medium particle is perpendicular to the direction of wave motion.
Transverse waves travel in the form of crests and troughs. The points of maximum displacement of medium particles are called troughs ( maximum displacement in upward direction ) and crests ( maximum displacement in downward direction ).
One trough and one successive crest together constitute one complete cycle of a wave.
Distance between top points of two successive troughs or bottom points of two successive crests give magnitude of wavelength.
Transverse waves involve changes in the shape of a medium. So they can travel through a medium which has fixed shape i.e. having rigidity.
It only occur in solids and free surfaces of liquids.

Longitudinal Wave Motion

In longitudinal wave, medium particles vibrate parallel to the direction of propagation of waves.

In transverse waves –

Displacement of medium particles is parallel to the direction of propagation of wave.
Longitudinal waves travel in the form of compression and rarefaction. Compression is the zone of high density of medium particles and rarefaction is the zone of lesser density of medium particles.
One complete compression and one complete rarefaction together constitute one complete cycle of a wave.
Distance between two similar points of two successive compression or rarefaction is the measure of wavelength.
Longitudinal waves involve changes in volume and density of medium. So they can travel through a medium which can compressed and sustain compression easily.
Longitudinal waves occur in solids, liquids and gases.

Types of Wave

Depending upon their nature, waves are classified into three categories –

Mechanical waves.
Electromagnetic waves.
Matter waves.

Mechanical Waves

Wave motions which require a material medium for their propagation are called mechanical waves or elastic waves.

Mechanical wave has following characteristics –

Mechanical wave requires elastic medium for propagation.
It depends on modulus of elasticity or Young’s modulus of medium.
It obeys Newton’s laws of motion.
Water, air, rock etc. are favorable mediums.
Water waves, sound waves, seismic waves etc. are examples of mechanical waves.

Electromagnetic Waves

Waves which require to travel in the form of oscillating and magnetic fields are called electromagnetic waves.

Electromagnetic wave has following characteristics –

Electromagnetic waves not require any material medium for their propagation.
It can travel through vacuum with the speed of light.
Visible light, ultraviolet rays, radio waves, micro waves and $X$ rays etc. are examples of electromagnetic waves.

Matter Waves

Matter waves or de-Broglie waves are the wave motion associated with motion of microscopic particles such as electrons, protons, neutrons, atoms and molecules etc.

A matter wave has following characteristics –

Matter waves are not electromagnetic in nature. It represents the probability of finding a particle in space.
Matter waves are independent of the charge on the material particle.
Electron microscope works on the basis of de-Broglie waves.
These are important for quantum theory of materials.

See numerical problems based on this article.