Waves

Light travels as waves. Waves can be described by their amplitude,wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength. 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. It is the medium that vibrates as the waves travel through.

As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top and bottom of a wave.

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 crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in 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 20kHz, radio stations broadcast radio waves with frequencies of about 100MHz, while most wireless computer networks operate at 2.4GHz.

The speed of a wave - its wave speed - is related to its frequency and wavelength, according to this equation:

wave speed (metre per second) = frequency (hertz) × wavelength (metre)

For example, a wave with a frequency of 100Hz and a wavelength of 2m travels at 100 × 2 = 200m/s.

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.