Introduction
Vibrations and sound
Energy and oscillations
Producing and measuring waves
Speed, frequency and length
Free waves and trapped waves
How do waves behave?
Classifying waves
Electromagnetic waves
  Radio waves
  Microwaves
  Infrared waves
  Light
  Ultraviolet rays
   X-rays
  Gamma rays
Mechanical waves

Introduction

If you twang several different lengths of ruler on the edge of a table, you will see them vibrate. The back and forth movements of rulers or tuning forks are called vibrations or oscillations. Just about everything can vibrate, from atoms to galaxies. Some of these vibrations can be picked up by the human senses.

Vibrations and sound

A vibrating ruler, a tuning fork and a guitar string all make sounds. All sounds are made by vibrations. A loudspeaker is designed to emit sound by making the surrounding air vibrate. You cannot see air, but you can get a sense of the vibrations by putting a table-tennis ball on top of a loudspeaker.

To describe vibrations scientifically, we need two words - frequency and amplitude. If you have ever pushed someone on a swing, you will know something about frequency and amplitude.

The number of complete swings each second (forwards from the position where you push and then all the way back again) is the frequency (measured in hertz, Hz). If you are getting the swing going, your pushes will have the same frequency as the swing.

The furthest distance that the swing goes from its middle position is the amplitude. The best place for you to stand is at a distance a little greater than the amplitude.

Energy and oscillations

A vibrating object has energy. The higher the frequency the more energy it has. One way of explaining this is to think of the average speed of the object. The higher the frequency, the higher the average speed and the higher the energy.

The bigger the amplitude the more energy a vibrating object has. The reason for this is that the amplitude also affects the average speed.

Oscillators such as a mass on a spring and a guitar string have kinetic and potential energy. Some of the potential energy comes from elastic stretching and squashing as the oscillator moves. When the spring or string is most stretched, the oscillator has its maximum amount of potential energy and no kinetic energy.

When the oscillator is moving at its fastest, it has its maximum amount of kinetic energy and no potential energy. All kinds of oscillators have energy. They can transfer their energy to their surroundings in the form of waves.

Producing and measuring waves

A vibrating object, or oscillator, can affect the things around it by sending out waves. If you throw a stone into the middle of a pond, the water in the middle oscillates up and down. This then affects water further away from the middle, and so on. What you see is a set of ripples, or waves, travelling away from the original disturbance caused by the stone. The stone gives energy to the water and the waves carry some of this energy away.

Waves come in many shapes and sizes. The size of a wave can be measured. To do this properly, two measurements must be made. They are the amplitude and the wavelength.

Waves come in different shapes and sizes Look at the waves above. Two of them have the same amplitude and two have the same wavelength. 1. Are waves A and C similar in some way? (A ruler might help you to give an accurate answer). 2.What about waves B and C?

Speed, frequency and length

Wavelength can be changed by two factors: the speed and the frequency of the wave. The connection between speed, frequency and wavelength is shown in the following equation:

wave speed = frequency x wavelength

V - f x ( is pronounced lambda. This is the Greek letter 'l'.)

Wave speed problems Here are two examples to give you practice at using the wave speed equation. 1. Imagine waves arriving at a seashore. They are 3 m apart, and two arrive every second. What is the speed of the waves? wavelength = 3 m frequency = 2 Hz So wave speed = 2 x 3 = 6 m s-1. 2. A penny whistle makes a sound that has a wavelength of 0.3 m. A tuning fork whose frequency is 1100 Hz produces the same note. What is the speed of the sound waves in air? wavelength = 0.3 m frequency = 1100 Hz So wave speed = 1100 x 0.3 = 330 m s-1.

Free waves and trapped waves

When you speak, the energy you use to make the sound travels away from you in all directions. The sound waves are free to travel. They are called progressive waves.

It is different with a guitar string. If you pluck a string, the sound waves travel up and down the string and bounce back off each end. The wave cannot escape from the string. A hollow-bodied guitar does allow some energy transfer from the strings to the air so that you can hear the note, but electric guitars hardly transfer any sound to the air.

How do waves behave?

As a wave travels along the surface of water, the water itself moves up and down. A piece of cork floating on a pond bobs up and down as a ripple travels past it. In this type of wave the vibrations are at right angles to the direction of the wave itself. These are called transverse waves ('trans' means 'across').

Electromagnetic waves are also transverse waves - the electric and magnetic fields causing the wave are at right angles to the direction of the wave itself.

The other kind of wave is a longitudinal wave. Here the vibrations are in the same direction as the wave. For example, sound waves, such as those produced by a tuning fork when it is struck, are longitudinal. The prongs of the fork and the air next to them vibrate backwards and forwards in the same direction as the wave.

Both types of wave are shown in the model Longitudinal and transverse waves.

Classifying waves

There are several types of wave:

• Tremors in the earth (earthquakes)
  
• Ocean waves
  
• Sound
  
• Light (including a special kind produced by lasers)
  
• Pulses in strings, ropes and springs
  
• Ripples in water
  
• Microwaves

You may think that waves come in so many different shapes and sizes, all with different speeds, that it would be impossible to group them together in families. However, the properties and behaviour of waves enables scientists to place waves into groups. Some properties are as follows:

Some waves travel more quickly than others
Some waves must have some type of material to travel through
Some waves can travel through empty space (a vacuum)

There is one very special family of waves, whose members are all able to travel through a vacuum all at the same speed. These are the electromagnetic waves.

Waves that cannot travel through a vacuum are often called mechanical waves. These need a material or medium to travel through. Examples of these are vibrations in the Earth (seismic waves), shock waves, sound and water waves.

Electromagnetic waves

The waves in this family have several things in common.
They all travel at the same speed in a vacuum. This is about 300 million m s^-1, the speed of light. It takes these waves just over a second to travel from the Earth to the Moon.
They are able to travel in a vacuum, though much of the time they are travelling through the Earth's atmosphere.
They can all have a heating effect when they are absorbed.
They are all produced by oscillating electrical and magnetic fields.

The frequency and wavelength of electromagnetic waves can be used to place them within a group known as the electromagnetic spectrum. The waves with a longer wavelength and lower frequency are shown on the left. Those with a shorter wavelength and higher frequency are shown on the right.

The electromagnetic spectrum

Radio waves

Radio waves are used to carry the radio and TV signals received in the home. Some manmade radio waves have a wavelength as long as 1,500 m (Radio 4) or even longer. Radio waves in outer space can have wavelengths of millions of kilometres. Television programmes are carried by waves of a shorter wavelength - about 0.5 m or less.

Transmitters are usually sited on tall masts on top of a hill so that their range is increased. Short-wave signals can also be reflected by a special layer of air in the Earth's atmosphere called the ionosphere.

Microwaves

Next in the spectrum are the microwaves. These have a wavelength of about 1 cm. These are used for heating things. The microwaves excite the particles inside the object, making them vibrate more quickly and so increasing their temperature. Microwaves are particularly efficient at heating liquids.

Microwaves are not just used in microwave ovens - they are also used to send messages. Satellites are often used to 'bounce' microwaves from one part of the world to another.

(a) A microwave has a wavelength of less than 1 cm (b) A radar wave may have a wavelength of about 5 cm.

Infrared waves

These are often called 'heat rays', but this is not a very accurate name since all electromagnetic waves can cause an increase in temperature. Infrared waves are given off by hot objects, including humans. 'Red-hot' objects, such as electric fires and cooker rings, produce a lot of infrared waves.

Light

Even hotter objects give off rays that people can see. These are known as visible rays. A filament in a light bulb becomes 'white hot' and gives off visible rays, which we know as light. The Sun produces light, as well as infrared waves, radio waves and ultraviolet waves.

Ultraviolet rays

These waves are not visible, but are well known for changing the colour of skin when people are exposed to them for long periods. These rays cause changes in the colouring or pigment of the skin. They can be quite dangerous since they may cause skin cancer.

Fortunately, most of the Sun's ultraviolet (UV) rays are absorbed by a layer in the Earth's atmosphere called the ozone layer. Many people are concerned that this protective layer is being weakened by some of the chemicals that now pollute the air. Most scientists agree that, in particular, gases produced by aerosol sprays (called CFCs) cause holes in the ozone layer.

One important use of UV rays is in producing long-life milk. UV rays of high frequency and high energy can be used to kill the bacteria in ordinary milk and keep it fresh for longer.

X-rays

Further along the electromagnetic spectrum lies a group of waves called X-rays. These can be used to photograph the inside of someone's body. Most of the X-rays pass through the patient's skin and flesh, while many of them are absorbed by bone.

The X-rays then pass on to a photographic plate sensitive to the rays (just as camera film is sensitive to visible light). After developing, the film shows up any cracks in the bone. X-rays can be used to examine the inside of many objects.

Gamma rays

Gamma rays are similar to X-rays, but they usually have a higher frequency, shorter wavelength and can penetrate much further into materials. Gamma rays come from the nucleus of an atom. These are the highest-energy rays and can be measured one at a time, using a Geiger counter. They pass through metals such as steel and can be used to detect cracks in metal objects. However, both X-rays and gamma rays can be extremely harmful to living things.

Mechanical waves

Many waves on Earth and in its atmosphere need a medium to travel through. These include:

• Sound waves
• Seismic waves
• Water waves