Progressive Waves – Longitudinal & Transverse waves

A progressive wave motion transmits energy from the source through a material or a vacuum without transferring matter. Wave motion can occur in many forms, such as water waves, sound waves, radio waves, light waves, and mechanical waves.

Waves: Basic Properties

Waves are produced by the oscillation of particles or electric and magnetic fields. They are defined by the following set of basic properties:

● Wavelength (λ) is the distance between any two successive corresponding points on the wave, for example between two maxima or two minima.

● Displacement (y) is the distance from the mean, central, undisturbed position at any point on the wave.

● Amplitude (a) is the maximum displacement from zero to a crest or a trough.

● Frequency ( f ) is the number of vibrations per second made by the wave. Frequency is measured in hertz (Hz). A frequency of 1 Hz is a rate of vibration of one oscillation per second. High frequencies are measured in kilohertz (kHz) (1 kHz = 1000 Hz) and megahertz (MHz) (1 MHz = 1 000 000 Hz).

● Period (T) is the time taken for one complete oscillation (T = 1/f ).

● Phase (ε ) is a term related to the displacement at zero time.

● Path difference is the difference in distance traveled by two waves from their respective sources to a common point.

● Speed (c) is a measure of how quickly energy is transmitted from place to place by wave motion.

Wave speed (c) = Frequency (f) × Wavelength (λ)

Phase and phase difference

The phase of a wave is related to the displacement of a specific point (say a crest) on the wave at zero time. The phase difference between two waves is the difference between the positions of the crests on the two waves.

When the positions of the crests and troughs of two waves coincide the waves are in phase. When the crests of one wave coincide with the troughs of the other the waves are out of phase (Figure 1).

In this case, the phase difference between the two waves is π radians, or 180°. Waves with a different phase difference would show a different shift along the time axis.

Figure 1: phase and phase difference

Longitudinal and Transverse Waves | Types of waves

Types of wave

Wave motion occurs as one of two types: longitudinal and transverse.

Longitudinal waves

In a longitudinal wave (Figure 2) the oscillation is along the direction of propagation of the wave, for example, sound waves and some mechanical waves.

figure 2: Longitudinal waves

In a longitudinal wave the particles of the material through which the wave is travelling move from side to side along the wave direction as the wave passes by. This oscillatory movement produces places of low pressure (rarefaction) and places of high pressure (compression). For this reason, a longitudinal wave is sometimes called a pressure wave.

Transverse waves

In a transverse wave (Figure 3) the oscillations are at right angles to the direction of propagation of the wave, for example, water waves, most electromagnetic waves, and some mechanical waves.

figure 3: Transverse waves


A wave in which the plane of vibration is constantly changing is called an unpolarised wave. When the vibrations of a transverse wave are in one plane only then the wave is said to be polarised (Figure 4).

figure 4: Polarization

It is important to realize that transverse waves can be polarised while longitudinal waves cannot. Therefore if a set of waves can be polarised it is very good evidence that these waves are transverse.

Applications of polarization

The uses of polarisation include polarising glasses for viewing 3D films, LCD displays, photographic filters, stress analysis investigation using transparent plastic specimens, and ‘Polaroid’ sunglasses, which reduce the glare from reflected sunlight. This last effect is due to the polarisation of reflected light from a surface.
Polarisation is also important for the transmission and reception of TV signals. The transmitting aerial and the receiving aerial must be aligned in the same direction for optimum signal reception.

Progressive Waves – Longitudinal & Transverse waves
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