Wave Properties

What is a wave?

A wave is a way of transferring energy from one place to another without transferring matter.

The particles of the material (the medium) only vibrate back and forth around where they started. The energy moves on.

• Sound waves carry energy from a speaker to your ear. The air does not blow into your ear.
• Ocean waves carry energy to the beach. A floating ball just bobs up and down.

Two types of waves

Transverse waves: the particles vibrate at right angles (perpendicular) to the direction the wave travels.

• Examples: waves on a string, ripples on water, light and all electromagnetic waves.
• They have crests (high points) and troughs (low points).

Longitudinal waves: the particles vibrate parallel to (along) the direction the wave travels.

• Examples: sound waves, a slinky pushed and pulled from one end.
• They have compressions (particles squeezed together) and rarefactions (particles spread apart).

Describing a wave

The wave equation

$$v=f\lambda$$

Speed = frequency × wavelength. And frequency and period are linked:

$$f=\frac{1}{T}T=\frac{1}{f}$$

Sound waves

Sound is a longitudinal wave and needs a medium (solid, liquid or gas). It cannot travel through a vacuum. In space, nobody can hear you shout.

• Speed of sound in air: about 340 m/s. Faster in water (about 1500 m/s) and faster still in steel (about 5000 m/s).
• Higher frequency = higher pitch.
• Bigger amplitude = louder.
• Humans hear from about 20 Hz to 20 000 Hz. Above that is ultrasound.

Quick summary

• Waves transfer energy without transferring matter.
• Transverse: vibrations at right angles to travel (light, water ripples).
• Longitudinal: vibrations along the direction of travel (sound).
• Wavelength $\lambda$, amplitude $A$, frequency $f$ (Hz), period $T$ (s).
• $v=f\lambda$ and $f=\frac{1}{T}$.
• Sound needs a medium. Pitch depends on frequency. Loudness depends on amplitude.

Quiz