Science

An introduction to 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.

# What are 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 wavesseismic waves: Shock waves travelling through the Earth, usually caused by an earthquake. are like this. They must travel through a medium. 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 radiationelectromagnetic radiation: Energy travelling as waves in the form of changing electrical and magnetic fields. are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.

# Amplitude, wavelength and frequency

You should understand what is meant by the amplitude, wavelength and frequency of a wave.

## Amplitude

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.

Amplitude and wavelength

## Wavelength

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.

## Frequency

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 wavesradio waves: Low frequency electromagnetic radiation used to transmit information such as television and radio programmes. with frequencies of about 100MHz, while most wireless computer networks operate at 2.4GHz.

Check your understanding of this section by having a go at this activity.

# Wave speed

You should know and be able to use the relationship between wave speed, frequency and wavelength.

## How fast do waves travel?

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.

Check your understanding of the equation by having a go at this activity.

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