Progressive waves distribute energy from a source to the surrounding area. Energy is moved as either vibrating particles or fields.

There are two types of progressive wave:

transverse waves: the displacement of the particles within the medium is perpendicular to the direction in which the wave is travelling
longitudinal waves: the displacement of the particles is in the same direction as that in which the
wave is travelling

Describing waves

Amplitude (A) is the maximum displacement of a particle within a wave from its position of equilibrium. It is
measured in metres (m).

The number of complete waves which pass a particular point in one second is known as the frequency (f).

This is measured in hertz (Hz).

The distance between two identical points on a full wave is called the wavelength (?). It is measured in metres (m).

The wave speed is measured in metres per second (ms-1).

Wavelength, frequency and wave speed are connected by the following equation:

c = f?

In which:
c = wave speed (ms-1)
f = frequency (Hz)
? = wavelength (m)

Phase

Points on a wave which always travel together in the same direction and rise and fall simultaneously are said
to be in phase with one another.

On the other hand, points on a wave which always travel in opposite directions and when one falls the other
rises are said to be in anti-phase with one another.

Path difference

The difference between the distance travelled by two waves is known as the path difference. This is measured in metres (m).