Longitudinal and transverse waves

Waves can be longitudinal or transverse.

Transverse waves

In transverse waves, the oscillations are at right angles to the direction of travel and energy transfer.

Light and other types of electromagnetic radiation are transverse waves. All types of electromagnetic waves travel at the same speed through a vacuum, such as through space.

Water waves and S waves are also transverse waves.

Longitudinal waves

In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer.

Sound waves and waves in a stretched spring are longitudinal waves. P waves are also longitudinal waves.

Longitudinal waves show areas of compression and rarefaction. In the slideshow below, the areas of compression are where the parts of the spring are close together, while the areas of rarefaction are where they are far apart.

Energy transfer and waves

Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd - the wave moves around the stadium, while each spectator stays in their seat, only moving up then down when it's their turn.

Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium, and it is the medium that vibrates as the waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves, and other types of electromagnetic radiation, are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.


The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the trough of one wave to the crest of the next wave. But it doesn't matter where you measure it - as long as it is the same point on each wave.


The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second.

The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies. For example:

  • most people cannot hear a high-pitched sound above 20 kHz
  • radio stations broadcast radio waves with frequencies of about 100 MHz
  • most wireless computer networks operate at 2.4 GHz

The plane mirror

Light waves reflect from surfaces. When waves reflect, they obey the law of reflection: the angle of incidence equals the angle of reflection.

Ray diagrams are drawn to explain reflection in a plane mirror (a flat mirror).

Constructing a ray diagram

In a ray diagram, the mirror is drawn as a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines.

The incident rays (the solid lines) should obey the law of reflection: the angle of incidence equals the angle of reflection.

Critical angle

Waves going from a dense medium to a less dense medium speed up at the boundary between them. This causes light rays to bend when they pass from glass to air at an angle other than 90°. This is refraction.

Beyond a certain angle, called the critical angle, all the waves reflect back into the glass. We say that they are totally internally reflected

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