# Uses of Waves

## Uses of Sound Waves

#### Echoes

Echoes are sound waves bouncing off surfaces. Sound waves obey the same first rule of reflection. (Remember: the angle of incidence is the same as the angle of reflection.)

The echo is usually quieter than the original noise as energy is lost as the wave travels along.

You can work out how far away something is using echo-sounds.

If it takes 20 seconds for the echo to be detected it must have taken 20 seconds for the sound to travel to the object and back. Using:

Distance = Speed x Time

The distance can be calculated. The speed of sound is 330 m/s so the calculation becomes:

Distance = 330 m/s x 20 s = 6600 m

This is the distance there and back, so the object is half that distance away, 3300 m.

Watch out. Many students forget to halve the distance.

Shiny hard surfaces reflect sound better than soft, surfaces. Bathrooms are good rooms to sing in as the sound bounces well off tiled walls. If you sing in the living room most of the sound energy is lost, because the energy is absorbed by the carpet, furniture and curtains.

#### Ultrasound

Sound waves that have a very high frequency are called ultrasound or ultrasonic waves. These sounds are so high that humans can't hear them. Dogs and bats have a higher hearing range than humans and can hear some ultrasonic waves.

Ultrasonic sound waves are made by electrical devices (like a loud speaker), which change electrical signals into sound waves.

There are many uses for ultrasound in medicine and industry. Here are some of them:

Looking at babies in the womb (pre-natal scanning):

A receiver compares the length of time it takes for the ultrasound waves to be detected. The longer the time it takes for the wave to reach the receiver the deeper into the body the wave has gone. This information is then used to build up a picture of the baby in the womb, which is then shown on a visual display, like a computer screen.

Cleaning instruments: Ultrasonic waves can be used to clean delicate instruments without having to take the equipment apart. The instrument is held in a liquid. The ultrasonic waves make the liquid particles vibrate at a high frequency, which cleans the surfaces of the equipment.

Detecting flaws and cracks in metal: This works in the same way as scanning babies in the womb. The ultrasonic waves bounce off different surfaces in the metal. The time it takes for the waves to bounce back to the receiver allows us to work out the depth the wave has travelled into the metal.

## Electromagnetic Spectrum

#### Waves in the spectrum

White light can be split up into lots of different coloured light waves using a prism. We call this range of colours the visible spectrum, however, this is only a small part of a very much longer spectrum called the electromagnetic spectrum. Humans can only see the visible part of the spectrum; the rest of the electromagnetic spectrum is invisible to the human eye. Other animals are able to detect other parts of the spectrum that human' s cannot see, for instance the infrared and ultraviolet waves.

All the parts of the electromagnetic spectrum can be reflected, refracted and diffracted which shows that each part is a type of wave. In fact, the electromagnetic spectrum is a whole continuous family of waves. They are all transverse waves. They all travel at the same speed through space. This speed is 300,000,000 m/s (the speed of light) this can also be written as 300 thousand km/s.

Electromagnetic waves do not need a medium (matter) to travel in. They can travel through space. This is fortunate as we get most of our energy from the sun as electromagnetic waves.

Although all the waves in the spectrum have the same speed, each type has its own range of frequencies and wavelengths. The difference in the frequencies gives each type of wave its own characteristics, for example, in the visible spectrum each different frequency has a different colour.

You will need to know the order of the spectrum. Remember gamma waves are at the high frequency end of the spectrum and radio waves are at the low frequency end.

Each type of wave in the spectrum has its own use. You will need to know the possible uses and dangers of each type of wave. Move the mouse over the spectrum to see the information about each wave:

The higher frequency radiations (U.V. X-rays and gamma waves) are the most dangerous. Low doses can cause normal cells to become cancerous. Higher doses will kill cells. This is because these types of radiation cause ionisation.

You should be able to describe ways of reducing the amount of radiation absorbed by people. The main safety points are:

• Point radiation sources away from people.
• Wear gloves and use tongs when handling substances that emit radiation.

#### Absorption and emission of waves - seeing colours

Why do different objects look different colours?

Each surface absorbs and reflects different frequencies of light. White surfaces reflect all colours. Black surfaces absorb all colours. Some people argue that black isn't really a colour at all; it's a lack of colour as all the light has been absorbed.

When any radiation is absorbed its energy is changed into heat energy. This means that black objects that absorb radiation easily heat up quickly, whereas white objects reflect the radiation so stay cooler.

A blue object reflects blue light and absorbs all the others. A red object reflects red light and absorbs the others and so on.

## Information Carrying

#### Types of signals

A rapidly increasing amount of people own mobile phones and there are very few houses in the U.K. that do not have some form of telecommunication. In the past all the systems were based on analogue signals but now the world is going digital.

Analogue signals

Analogue signals cover a whole continuous range of values.

Digital signals

Digital signals have only two values off (0) and on (1).

Digital signals have two main advantages:

• They are a higher quality than analogue signals. This is because they are not changed as much as analogue signals when they are transmitted. (Transmitted means moved or transferred from one point to another.)
• More information can be sent as a digital signal, in a certain length of time, compared with analogue signals.
• When signals are transmitted they lose energy so signals are amplified to increase the energy again. Signals can also pick up extra, unwanted signals. This is called noise.

For analogue signals the different frequencies in the signal lose different amounts of energy. When the signal is amplified these differences and any noise are also amplified. This makes the signal deteriorate and become less and less like the original signal.

This problem is less noticeable with digital signals, as the on and off states are easy to see, even with some noise added. The quality of the signal is less affected by the transmission.

#### Optical fibres versus copper cables

When telecommunications began signals were sent from house to house along copper cables, via switchboards. Now optical fibres have replaced many of these copper cables.

Electrical signals have to be used in copper cables, whereas electromagnetic waves can be used in optical fibres. By using digital signals, information can be sent along the optical fibres as pulses of light.

There are three main advantages of using optical fibres rather than coppercables:

• Optical fibres allow much faster transmission (delivery) of the signals than copper cables.
• Optical fibres can also carry far more information than a copper cable that is the same diameter.
• Light pulses lose less energy as they travel along an optical fibre than an electrical signal in a copper cable.

#### Changing waves into electrical signals

Waves have energy. This energy can be changed into other forms. For instance, when electromagnetic waves are absorbed by metal, the energy that is absorbed can be changed into an electrical signal as an alternating current. Match the correct devices to the wave form changes below.

Sound waves can also be converted to electrical signals, using a microphone. This is how speech and music can be transmitted long distances along cables. The electrical signals can then be converted to electromagnetic waves that can be sent along optical fibres or to satellites.

If waves are converted to electrical signals so that the frequency and amplitude of the electrical signal match the frequency and amplitude of the wave being converted, the electrical signal is an analogue signal. This signal can then be converted to a digital signal.

## Exam-style Questions

1. The diagram shows a representation of a sound wave on the screen of an oscilloscope.

a) Name a device that could have been used to capture the sound wave and turn it into the electrical signal for the oscilloscope.

(1 mark)

b) How would the trace change if the frequency of the sound increased?

(2 marks)

c) Each square represents 2 x 100ths of a second.

How long does it take for one wave to happen?

(2 marks)

d) How many waves happen in one second?

(1 mark)

e) What is the frequency of the sound wave?

(2 marks)

f) If sound travels at a speed of 330 m/s, then what is the wavelength of the wave?

(2 marks)

(Marks available: 10)

Answer outline and marking scheme for question: 1

a) Microphone.

(1 mark)

b) The wave should become more compressed but the wave height remains the same.

(2 marks)

c) 0.08 s

(2 marks)

d) 12.5 waves

(1 mark)

e) 12.5 Hz

(2 marks)

f Wavelength = wave speed / frequency

Wavelength = 330 / 12.5 = 26.4 m

(2 marks)

(Marks available: 10)

2. a) A ray of light approaches the rear view mirror in a car at an angle of 33 degrees to the normal line.

What is the normal line?

(2 marks)

b) What would be the size of the angle of reflection?

(1 mark)

c) If the mirror is moved to position marked X how would the path of the reflected ray of light be effected?

(1 mark)

d) What would the driver see?

(1 mark)

e) If the driver wished to check her make up in what direction might she adjust the rear view mirror?

(1 mark)

f) This diagram shows the arrangement of two mirrors in a periscope.

Copy and complete the diagram to show the ray of light passing through the periscope.

(2 marks)

g) State a use for a periscope.

(1 mark)

h) What other optical device could be used instead of mirrors in a periscope?

(1 mark)

(Marks available: 10)

Answer outline and marking scheme for question: 2

a) Imaginary line at right angles to the mirror from which all angles are measured.

(2 marks)

b) 33 degrees.

(1 mark)

c) The ray would reflect upwards.

(1 mark)

d) The driver would see the ceiling.

(1 mark)

e) Tilt the top downwards.

(1 mark)

f)

Light rays at 45 degrees to mirrors, with direction marked correctly.

(2 marks)

g) Any valid use, for example, looking over walls. Driver seeing upper deck on double decker buses.

(1 mark)

h) Prisms.

(1 mark)

(Marks available: 10)

3. Some waves approach a beach.

a) Why do the waves get closer as the waves approach the beach?

(2 marks)

b) If a large rock is placed at X, then what will happen to the waves that pass it?

(2 marks)

c) Nearby the waves pass through a gap between two pier supports. Describe what happens to the waves as they pass through.

(2 marks)

d) Ashia has a glass of lemonade and she notices that the straw looks crooked.

Use ideas of refraction to explain this.

(2 marks)

e) Name two optical devices that work because of refraction.

(2 marks)

(Marks available: 10)

Answer outline and marking scheme for question: 3

a) They slow down because the water is getting shallower - refraction.

(2 marks)

b) Waves diffract as they pass curving around into the space behind the rock.

(2 marks)

c) Waves diffract as they go between the supports spreading into a curved wave.

(2 marks)

d) Light rays from the straw in the air come straight to the eye. Rays from the straw in the water are refracted as they leave the water. This change of direction means they appear to come from somewhere else.

(2 marks)

e) Lens, Prism.

(2 marks)

(Marks available: 10)

4. The diagram shows the path of light rays entering the eye of a fish.

a) Why does the ray P bend as it enters the water?

(1 mark)

b) What has happened to ray Q as it entered the water?

(1 mark)

c) What is the name of the effect shown by rays N and S?

(1 mark)

d) This effect is used in Fibre Optic communication.

Why does the ray of light not escape the optical fibre?

(2 marks)

e) Fibre optic cables are replacing copper wires in telecommunication links.

Give two advantages fibre optic cables have over copper wire.

(2 marks)

f) Give two other uses for fibre optics.

(2 marks)

(Marks available: 9)

Answer outline and marking scheme for question: 4

a) It refracts - a decrease in speed makes it bend inwards.

(1 mark)

b) It slows but does not deviate because it met the boundary at right angles.

(1 mark)

c) Total internal reflection.

(1 mark)

d) It meets the boundary at an angle greater than the critical angle.

(2 marks)

e) Greater data capacity, fewer signal repeaters, smaller and lighter, cheaper raw materials, more secure data transmission.

(2 marks)

f) Endoscopy, intranet links, novelty items.

(2 marks)

(Marks available: 9)

5. Here is a diagram of the electromagnetic spectrum:

a) What are the missing parts of the electromagnetic spectrum labeled P and Q?

(2 marks)

b) Which end of the spectrum has the longest wavelength?

(1 mark)

c) Which end of the spectrum has the highest energy?

(1 mark)

d) Give one use for the infrared.

(1 mark)

e) Name two detectors of visible light.

(2 marks)

f) Ultra Violet radiation is present in sunlight.

In what way is the Ultra Violet harmful to humans?

(1 mark)

g) How can the risks be reduced?

(2 marks)

(Marks available: 10)

Answer outline and marking scheme for question: 5

a) P-microwaves, Q-xrays.

(2 marks)

(1 mark)

c) Gamma rays.

(1 mark)

d) Heat therapies, thermal imaging cameras.

(1 mark)

e) The eye, photographic film, ldr, photo voltaic cell.

(2 marks)

f) Causes sunburn and skin cancers.

(1 mark)

g) Wearing sunscreen and limiting time in the sun.

(2 marks)

(Marks available: 10)

6. During pregnancy the unborn foetus is monitored using Ultrasound.

a) What is Ultrasound?

(1 mark)

b) Name one animal that might hear ultrasound.

(1 mark)

c) Why would the doctor not use x-rays to monitor the unborn child?

(2 marks)

Ultrasound can also be used to detect shoals of fish, it can be called sonar.

A pulse of ultrasound is sent out from the boat an echo is heard 0.3 seconds later.

d) What is an echo?

(1 mark)

e) The speed of ultrasound in water is 1500 m/s.

Why does ultrasound (and sound) travel faster in water than in air?

(2 marks)

f) Calculate the depth of the shoal of fish.

(3 marks)

(Marks available: 10)

Answer outline and marking scheme for question: 6

a) Sound vibrations too high for humans to hear.

(1 mark)

b) Bats, dogs...

(1 mark)

c) X-rays would harm the developing foetus.

(2 marks)

d) An echo is wave a sound wave reflects off something.

(1 mark)

e) The particles in a liquid are closer allowing better interaction between particles in the longitudinal wave.

(2 marks)

f) Speed = distance / time. Distance = speed x time.

Distance = 1500 x 0.3 = 450 m

Half this to get depth.

Depth = 225 m.

(3 marks)

(Marks available: 10)

7. a) What does a seismometer measure?

(1 mark)

b) Name the scale that describes earthquake intensity.

(1 mark)

c) An earthquake creates S-waves and P-waves.

Which of these is a longitudinal wave and which is a transverse wave?

(2 marks)

d) How is the motion different in a longitudinal and a transverse wave?

(2 marks)

e) Which of S-waves and P-waves can travel through liquids?

(1 mark)

f) On the opposite side of the earth to the earthquake, only one type of wave is detected.

Which type is detected?

(1 mark)

g) What does this tell us about the core of the earth?

(2 marks)

(Marks available: 10)

Answer outline and marking scheme for question: 7

a) Vibrations in the earth, earthquakes.

(1 mark)

b) Reichter scale

(1 mark)

c) S-waves are transverse and p-waves are longitudinal.

(2 marks)

d) Transverse motion has particles oscillating across the direction of travel, whereas in longitudinal the oscillations are parallel to the direction of travel.

(2 marks)

e) P-waves.

(1 mark)

f) P-waves.

(1 mark)

g) The core is liquid as the waves that get through can go through liquid.

(2 marks)

(Marks available: 10)

## Sound Waves

Sound waves are longitudinal waves, made by particles vibrating. These vibrations are passed along to nearby particles, which then pass them on again. This is how sound waves travel along through solids, liquids and gases. When the particles vibrate near your eardrum, your eardrum vibrates. This movement gets turned into an electrical signal, which is then passed on to your brain.

Sound waves need particles to travel along, so they cannot travel in space, or any other vacuum. You can see the sun, but you can't hear the massive explosions that are taking place there, as light can travel in space but sound can't.

Sound can be reflected, refracted and diffracted which shows that it travels as a wave. Sound waves are longitudinal waves

The characteristics of sound waves decide the pitch and loudness of the sound.

#### Pitch

The pitch of a note is how high the note is. A man's voice tends to have a lower pitch than a woman's voice. Bats makes such a high-pitched noise that humans find it hard to hear.

• The pitch of a note depends on the frequency of the wave.
• The higher the frequency of the wave the higher the note.
• The shorter the wavelength of the wave the higher the note.

Click on the arrows to increase or decrease the pitch:

#### Loudness

How quiet a sound is depends on the amplitude of the sound wave.

The greater the amplitude of the wave the louder the sound.

Click on the arrows to increase or decrease the loudness:

#### Oscilloscope traces

In describing waves the terms amplitude, time period and frequency were explained. Most waves can be made into electrical signals. We can use oscilloscopes to look at these electrical signals, which represent the waves, on a screen. We can then measure the characteristics of the wave on the screen.

Measuring amplitude

An oscilloscope has a scale on it that tells you what the height of each square is equivalent to. In the diagram above, the height of one square is equal to 1 cm. This is written as 1cm/division. As the amplitude of the wave is 3 squares high the amplitude of the wave is 3 cm.

Worked examples

Measuring time period (the time for one wave)

The oscilloscope also has a 'time-base scale'. This tells you the scale across the screen. In the diagram above, each square across is equal to 2 seconds. As the wave is 4 squares long the time period of the wave is 2 x 4 = 8 seconds.

Worked Examples

Don't forget, the longer the time period the lower the frequency.

If the time-base scale stays the same it is easy to compare wave frequencies by looking at how many waves are on the screen, for example:

The more waves that fit on the screen the higher the frequency must be.

## S-Cool Revision Summary

Sound waves are caused by particles vibrating. The frequency of the vibration decides the pitch of the sound. The amplitude of the vibrations decides the loudness of the sound.

Ultrasound waves are high frequency sound waves, which are beyond the human hearing range. Ultrasound is used for seeing babies in the womb, detecting cracks in metal and cleaning instruments.

Waves can be represented on an oscilloscope screen, which can be used to measure the characteristics of the waves. You should be able to find the amplitude and time period of a wave from an oscilloscope screen.

The electromagnetic spectrum is a series of waves that all travel at the same speed in a vacuum. They are all transverse. Each part of the spectrum has different uses and dangers. Each part of the spectrum has a different frequency and wavelength. Gamma waves are at the high frequency end of the spectrum. Radio waves are at the low frequency end. You will need to know the uses and dangers of each part of the spectrum.

Different surfaces and materials absorb different frequencies of waves. White surfaces reflect most waves. Black surfaces absorb most waves.

Information can be carried along copper cables as electrical signals, or along optical fibres as electromagnetic wave pulses.

Optical fibres have advantages over copper cables. Optical fibres can carry more information, the signals can travel faster and lose less energy as they travel along the cable.

There are two types of signals, analogue and digital. Analogue signals have a continuous range of values. Digital signals have only two values, on (1) and off (0).

Digital signals have advantages over analogue signals. Digital signals are easier to transmit as they are less affected by noise, it is also possible to send more information, in a certain time, as a digital signal than as an anologue signal.