Transverse 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, and then down, when it is 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 [seismic waves: vibrations caused by earth movements ] are like this: they must travel through a medium. It is the medium that vibrates as the waves travel through it.

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 [electromagnetic radiation: Energy travelling as waves in the form of changing electrical and magnetic fields. ], are like this. They can travel through empty space. Electrical or magnetic fields vibrate as the waves travel through them.

Transverse waves

All types of electromagnetic radiation, including visible light, are transverse waves waves. In transverse waves the vibrations are at right angles to the direction of travel, as seen in the animation below of a wave sent down a piece of string. The hand moves up and down, as does each part of the string. But the wave moves along from left to right. Water waves and s-waves - one type of seismic wave - are also transverse waves.

For your exam you need to know that lasers produce an intense narrow beam of light.

Features of waves

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 waves [radio 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 (m/s) = frequency (hertz, Hz) × wavelength (metre, m)

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.

Morse code

Morse code was invented by Samuel Morse in 1832. Letters and numbers are represented by a series of dots and dashes - a dash lasts three times longer than a dot.

The Morse code

Morse code can be used to transmit signals in many different ways, including visible light, radio waves and electrical pulses.

Sending information using electrical pulses requires wires, unlike visible light and radio waves, which are wireless. The use of visible light needs the transmitter and receiver to be in the line of sight. But it is more secure against eavesdroppers than radio waves.

Now try a Test Bite - foundation

Lasers - higher

Lasers

A laser - 'Light Amplification by Stimulated Emission of Radiation' - produces an intense narrow beam of light. A typical laser beam might only spread out by 1m when shone onto a surface 1km away. The light waves in a laser beam are:

• the same frequency
• in phase with each other

When light waves are in the same phase, their crests all line up, as do their troughs.

CD players

Laser beams are used to read information from CDs in CD players. The shiny surface of a CD contains microscopic pits in a spiral track. The pattern of pits contains digital information. The laser light is reflected off the surface of the CD as the disk spins, and is detected:

• the change in the amount of reflected light caused by entering or leaving a pit is interpreted as a 1
• no change in the amount of reflected light is interpreted as a 0

Now try a Test Bite - higher