Light and sound travel as waves.

There are two types of wave - transverse and longitudinal. 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.

Visible light, infrared rays, microwaves and other types of electromagnetic radiation are like this. They can travel through empty space. Electrical or magnetic fields vibrate as the waves travel through.

Light and other types of electromagnetic radiation are transverse waves.

Water waves and S waves (a type of seismic wave) are also transverse waves.

In transverse waves, the vibrations are at right angles to the direction of travel.

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

In longitudinal waves, the vibrations are along the same direction as the direction of 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. It is the distance from the middle to the top.

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).

The speed of a wave - its wave speed (metres per second, m/s)- is related to its frequency (hertz, Hz) and wavelength (metre, m), according to this equation:

wave speed = frequency x wavelength

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

The speed of a wave does not usually depend on its frequency or its amplitude.

Light and sound both travel as waves, but they are not identical. The table summarises the similarities and differences between them.

PropertyLightSound

Type of wave

Transverse Longitudinal

Can they travel through a vacuum?

Yes No. Sound waves can only pass through a solid, liquid or gas

Can they be reflected?

Yes Yes

Can they be refracted?

Yes Yes

Can they be diffracted?

Yes Yes

Can they interfere?

Yes Yes

Sound waves and light waves reflect from surfaces. Remember that they behave just like water waves in a ripple tank. The angle of incidence equals the angle of reflection

Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.

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

Beyond a certain angle, called the critical angle

The critical angle for most glass is about 42 °.

An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.

When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is diffraction.

The amount of diffraction depends on the wavelength and the size of the gap.

The extent of the spreading depends on the width of the gap compared with the wavelength of the waves. The smaller the width of the gap compared with the wavelength of the wave, the stronger the diffraction.

Where two waves meet, their effects are added together. This is called interference. When they arrive in step, they reinforce each other to give a wave of greater amplitude.This is called constructive interference.

When they arrive out of step, they cancel out. This is called destructive interference.

Interference of light is when a laser is shone on two slits very close together, the diffracted beam can be seen on a screen to have bright and dark bands on it.

White light can be split up to form a spectrum using a prism. This is a block of glass with a triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light the more it is refracted.

Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through the vacuum of empty space.

FrequencyType of electromagnetic radiationTypical useWavelength highest gamma radiation killing cancer cells shortest X-rays medical images of bones ultraviolet radiation sunbeds visible light seeing infrared radiation optical fibre communication microwaves cooking lowest radio waves television signals longest

Electromagnetic waves do not need a substance to travel through. They can travel through the vacuum of empty space. In a vacuum, they all travel with the same very high speed: 300 000 000 m/s.

A beam of electromagnetic radiation delivers energy in ‘packets’ called photons.

The energy delivered by each photon increases with the frequency of the electromagnetic waves. This means that gamma photons have the most energy, and radio photons the least.

The intensity of a beam of electromagnetic radiation is the energy it delivers to a surface each second. This depends on how much energy each photon delivers, and on how many photons per second arrive at that surface.

Digital signals maintain their quality better than analogue signals.

Noise

All signals become weaker as they travel long distances. They may also pick up random extra signals. This is called noise.

Analogue signals: Noise adds extra random information to analogue signals. Each time the signal is amplified the noise is also amplified. Gradually, the signal becomes less and less like the original signal. They have a range of values.

Digital signals: Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the 'on' states of the digital signal. As a result, the electronics in the amplifiers can ignore the noise and it does not get passed along. This means that the quality of the signal is maintained.

Music and speech vary continuously in frequency and amplitude. In the same way, analogue signals can vary in frequency, amplitude or both.

In Amplitude Modulation, the radio wave varies in amplitude to match the changes in the sound wave.

Digital signals are a series of pulses consisting of just two states, ON (1) or OFF (0). There are no values in between. The sound is converted into a digital code of 0s and 1s, and this coded information controls the short bursts of waves produced by a source. DAB radio is Digital Audio Broadcast radio – it is transmitted as digital signals.

Digital signals carry more information per second than analogue signals. This is the same whether optical fibres, cables or radio waves are used. Digital signals maintain their quality over long distances better than analogue signals. You will notice far less noise