Waves: sound, light and the electromagnetic spectrum
By the end of this chapter you will be able to
- Describe sound as a longitudinal wave, and say why it needs a medium
- Use the frequency range of human hearing and the link between frequency and pitch
- Apply the law of reflection to a ray diagram, and describe an image in a plane mirror
- Describe refraction in simple terms, for a ray entering a denser medium
- Order the electromagnetic spectrum, and give a use and a hazard for each type
You should already know
- Energy and heating (Year 8 Chapter 11)
- Measurements and angles (Year 8 Chapter 1 and Year 7 Science)
Where this leads
- Forces and motion (Year 8 Chapter 13)
National Curriculum for England, Key Stage 3 science, Year 8. Content: 8P3 sound as longitudinal vibrations; sound needs a medium and travels at different speeds in solids, liquids and gases; the range of human hearing; frequency and pitch, 8P4 light: reflection and the law of reflection, plane mirrors and ray diagrams; refraction (basic), 8P5 the electromagnetic spectrum: its order, uses and hazards (basic); all electromagnetic waves travel at the same speed in a vacuum.
Exercise P Do you remember?
Method. Recall the word.
- Recall. The energy of a moving object is kinetic energy.kinetic
- Write the answer. kinetic
Check. Speed affects kinetic energy. ✓
Method. Divide.
- Divide. Share 180 into 6 equal parts.180 / 6
- Calculate. Each part is 30.180 / 6 = 30
- Write the answer. 30
Check. 30 x 6 = 180. ✓
Method. Recall the word.
- Recall. The line at right angles to a surface is the normal.normal
- Write the answer. normal
Check. Angles are measured from the normal. ✓
Method. Recall the unit.
- Recall. Frequency is measured in hertz, with the symbol Hz.hertz
- Write the answer. hertz
Check. One hertz is one cycle per second. ✓
Method. Multiply.
- Multiply. 100 million is 100 000 000.100 000 000
- Calculate. Three times that is 300 000 000.3 x 100 000 000 = 300 000 000
- Write the answer. 300000000
Check. 300 000 000 divided by 3 is 100 000 000. ✓
Sound as a longitudinal wave
Sound is a longitudinal wave. The particles of the medium vibrate parallel to the direction the wave travels. The wave makes regions where the particles are crowded, called compressions, and regions where they are spread out, called rarefactions.
Sound needs a medium to travel, so it cannot pass through a vacuum. It travels fastest in solids, more slowly in liquids, and most slowly in gases.
The table gives the approximate speed of sound in three media.
| Medium | Approximate speed of sound (m/s) |
|---|---|
| air | 340 |
| water | 1500 |
| steel | 5000 |
Key ideas
- Sound is a longitudinal wave: particles vibrate parallel to the direction of travel.
- Sound needs a medium, so it cannot travel through a vacuum.
- Sound travels faster in solids than in liquids, and faster in liquids than in gases.
- Compressions are crowded; rarefactions are spread out.
Method. Recall the fact.
- Recall. Sound is carried by the vibration of particles in a medium.particles
- Decide. So sound needs a medium; the answer is yes.yes
- Write the answer. yes
Check. Sound cannot cross a vacuum. ✓
Method. Find the largest speed in the table.
- Look. The speeds are 340 m/s in air, 1500 m/s in water and 5000 m/s in steel.5000
- Name it. The largest speed is in steel.steel
- Write the answer. steel
Check. Solids carry sound faster. ✓
Method. Recall the type.
- Recall. Particles vibrating parallel to the direction of travel is a longitudinal wave.longitudinal
- Name it. So the type is a longitudinal wave.longitudinal wave
- Write the answer. longitudinal wave
Check. Sound is one example. ✓
Watch out
Watch out: in a longitudinal wave the particles do not travel with the wave. They vibrate back and forth about their positions.
Exam tip
Exam tip. Say “the particles vibrate parallel to the direction of the wave”, and name the medium the sound travels through.
Try it (with your teacher, or a grown-up at home). Look at the longitudinal wave picture. Mark one compression and one rarefaction. Explain in one sentence why sound cannot travel through space. Do not listen for any particular sound; use the picture.
Exercise 12A Sound as a longitudinal wave
Method. Recall the type.
- Recall. Sound is a longitudinal wave.longitudinal wave
- Write the answer. longitudinal wave
Check. Particles vibrate parallel to the direction of travel. ✓
Method. Recall the direction.
- Recall. In a longitudinal wave, particles vibrate parallel to the direction of travel.parallel
- Write the answer. parallel
Check. Transverse waves vibrate at right angles. ✓
Method. Recall the word.
- Recall. A crowded region of a longitudinal wave is a compression.compression
- Write the answer. compression
Check. The spread-out region is a rarefaction. ✓
Method. Recall the word.
- Recall. A spread-out region of a longitudinal wave is a rarefaction.rarefaction
- Write the answer. rarefaction
Check. Compressions and rarefactions alternate. ✓
Method. Recall the fact.
- Recall. Sound needs a medium, and a vacuum has no medium.no medium
- Decide. So the answer is no.no
- Write the answer. no
Check. Space is a vacuum. ✓
Method. Read the table.
- Read. The table gives the speed in water as 1500 m/s.1500
- Write the answer. 1500
Check. Water is denser than air. ✓
Method. Read the table.
- Read. The table gives 340 m/s for air.340
- Write the answer. 340
Check. Gases are the slowest medium. ✓
Method. Compare the table.
- Compare. Air is 340 m/s and water is 1500 m/s.340 and 1500
- Decide. So sound travels more slowly in air.air
- Write the answer. air
Check. Liquids carry sound faster than gases. ✓
Method. Use the general pattern.
- Recall. Particles in a solid are closer together, so vibrations pass on faster.closer together
- Decide. So the statement is true; the answer is yes.yes
- Write the answer. yes
Check. Steel is faster than air in the table. ✓
Method. Check the medium.
- Check. Sound needs a medium, and space is a vacuum.medium
- Decide. So Ali is not right; the answer is no.no
- Write the answer. no
Check. Being a wave does not mean it needs no medium. ✓
Method. Check what moves.
- Check. The particles vibrate about their positions; the energy moves along.energy moves
- Decide. So the particles do not travel with the wave; the answer is no.no
- Write the answer. no
Check. Energy is transferred, not the particles. ✓
Method. Think about the particles.
- Think. The particles in a solid are close together and touching.close together
- Conclude. So the vibration passes from one particle to the next more quickly.more quickly
- Write the answer. Accept any sentence that says the particles in a solid are closer together, so they pass the vibration on more quickly.
Check. Closer particles pass on vibrations faster. ✓
Method. Think about the medium.
- Think. Sound needs a medium to carry it.medium
- Conclude. The Moon has no medium, so sound cannot travel to a listener.no medium
- Write the answer. Accept any sentence that says the Moon has no air or other medium, so sound cannot travel to the ear.
Check. A vacuum has no medium. ✓
Frequency, pitch and the range of hearing
The frequency of a wave is the number of complete waves passing a point each second. It is measured in hertz (Hz). A higher frequency gives a higher pitch. The amplitude of a wave is the size of its vibration; a larger amplitude gives a louder sound.
Humans can hear sounds with frequencies from about 20 Hz to 20 000 Hz. Sounds above this range are ultrasound; sounds below it are infrasound.
Key ideas
- Frequency is measured in hertz (Hz), the number of waves each second.
- Higher frequency means higher pitch.
- Amplitude is the size of the vibration; larger amplitude means louder.
- Humans hear about 20 Hz to 20 000 Hz.
Method. Compare with the range.
- Compare. The upper limit of human hearing is about 20 000 Hz.20 000 Hz
- Decide. So a sound at 20 000 Hz is at the upper edge; the answer is yes.yes
- Write the answer. yes
Check. At the top end of the range. ✓
Method. Compare with the range.
- Compare. Above about 20 000 Hz, a sound is ultrasound.above 20 000
- Decide. So 40 000 Hz is ultrasound; the answer is yes.yes
- Write the answer. yes
Check. Ultrasound is above human hearing. ✓
Method. Higher frequency means higher pitch.
- Compare. The frequency of 2000 Hz is higher than 200 Hz.2000
- Decide. So the higher pitch is 2000 Hz.2000
- Write the answer. 2000 Hz
Check. Ten times the frequency. ✓
Watch out
Watch out: pitch depends on frequency, not on amplitude. A quiet sound can have a high pitch.
Exam tip
Exam tip. Say “higher frequency gives higher pitch”, and give the unit hertz.
Try it (with your teacher, or a grown-up at home). Look at the frequency range and say whether each of these frequencies is in the range of human hearing: 50 Hz, 5000 Hz, 50 000 Hz. Do not play or listen to any sound in class; answer from the numbers.
Exercise 12B Frequency, pitch and hearing
Method. Recall the unit.
- Recall. Frequency is measured in hertz.hertz
- Write the answer. hertz
Check. The symbol is Hz. ✓
Method. Recall the symbol.
- Recall. The symbol for hertz is Hz.Hz
- Write the answer. Hz
Check. Two letters, the first capital. ✓
Method. Recall the lower limit.
- Recall. Humans hear from about 20 Hz.20 Hz
- Write the answer. 20
Check. Lower sounds are infrasound. ✓
Method. Recall the upper limit.
- Recall. Humans hear up to about 20 000 Hz.20 000 Hz
- Write the answer. 20000
Check. Higher sounds are ultrasound. ✓
Method. Compare with the range.
- Compare. Five thousand is between 20 and 20 000.between
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. It is in the middle of the range. ✓
Method. Compare with the range.
- Compare. Fifty is above 20 and below 20 000.above 20
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. It is in the lower part of the range. ✓
Method. Compare with the range.
- Compare. Fifty thousand is above the upper limit of 20 000.above 20 000
- Decide. So it is ultrasound; the answer is yes.yes
- Write the answer. yes
Check. Ultrasound is above human hearing. ✓
Method. Higher frequency means higher pitch.
- Compare. The frequency 1000 Hz is higher.1000
- Decide. So 1000 Hz has the higher pitch.1000
- Write the answer. 1000
Check. Higher frequency, higher pitch. ✓
Method. Recall the word.
- Recall. The size of the vibration is the amplitude.amplitude
- Write the answer. amplitude
Check. A larger amplitude gives a louder sound. ✓
Method. Recall the difference.
- Recall. Pitch depends on frequency, and loudness depends on amplitude.frequency
- Decide. So amplitude does not change pitch; the answer is no.no
- Write the answer. no
Check. Loudness and pitch are separate. ✓
Method. Check the difference.
- Check. Quiet means a small amplitude; pitch depends on frequency.small amplitude
- Decide. So Ali is not right; the answer is no.no
- Write the answer. no
Check. Loudness and pitch are separate properties. ✓
Method. Frequency is the waves per second.
- Read. The frequency is the number of waves each second.500 waves per second
- Write. So the frequency is 500 Hz.500
- Write the answer. 500
Check. The unit is hertz. ✓
Method. Compare with the range.
- Think. Human hearing extends up to about 20 000 Hz.up to about 20 000
- Conclude. So 30 000 Hz is above the range and cannot be heard.above the range
- Write the answer. Accept any sentence that says 30 000 Hz is above the upper limit of human hearing, about 20 000 Hz, so it is ultrasound.
Check. Ultrasound is outside hearing. ✓
Method. Subtract the frequencies.
- Compare. The 4000 Hz sound has the higher pitch.4000
- Calculate. The difference is 4000 minus 2000.4000 - 2000 = 2000
- Write the answer. 2000
Check. 2000 + 2000 = 4000. ✓
Reflection of light
Reflection is the bouncing of light off a surface. The normal is a line drawn at right angles to the surface at the point of contact. The angle of incidence is the angle between the incoming ray and the normal. The angle of reflection is the angle between the reflected ray and the normal.
The law of reflection says: the angle of incidence equals the angle of reflection. Both angles are measured from the normal, not from the surface.
The table gives class data for three incident rays on a plane mirror.
| Angle of incidence (degrees) | Angle of reflection (degrees) |
|---|---|
| 20 | 20 |
| 35 | 35 |
| 50 | 50 |
Key ideas
- The normal is a line at right angles to the surface.
- The angle of incidence equals the angle of reflection.
- Both angles are measured from the normal.
- A plane mirror forms a virtual image behind the mirror.
Method. Use the law of reflection.
- Recall. The angle of reflection equals the angle of incidence.equals
- Calculate. So the angle of reflection is 35 degrees.35
- Write the answer. 35 degrees
Check. The table shows the same rule. ✓
Method. Use the law of reflection.
- Recall. The two angles are equal.equal
- Calculate. So the angle of incidence was 50 degrees.50
- Write the answer. 50 degrees
Check. Both angles are measured from the normal. ✓
Method. Recall the image.
- Recall. The image seems to be behind the mirror, where no light actually goes.behind the mirror
- Decide. An image that cannot be formed on a screen is virtual.virtual
- Write the answer. virtual
Check. Virtual images cannot be caught on a screen. ✓
Watch out
Watch out: the angles are measured from the normal, not from the mirror surface. An angle of 30 degrees to the mirror is 60 degrees to the normal.
Exam tip
Exam tip. Draw the normal first, dashed and at right angles. Then measure both angles from it.
Try it (with your teacher, or a grown-up at home). With a grown-up, use a small plane mirror and a torch to test the law of reflection on paper. Draw the normal and the ray, and measure both angles. Do not shine any torch at a person’s eyes.
Exercise 12C Reflection of light
Method. Recall the word.
- Recall. The line at right angles to the surface is the normal.normal
- Write the answer. normal
Check. Angles are measured from it. ✓
Method. Recall the words.
- Recall. The angle between the incoming ray and the normal is the angle of incidence.angle of incidence
- Write the answer. angle of incidence
Check. The reflected angle is the other one. ✓
Method. Recall the words.
- Recall. The angle between the reflected ray and the normal is the angle of reflection.angle of reflection
- Write the answer. angle of reflection
Check. The angles are equal. ✓
Method. Recall the law.
- Recall. The angle of incidence equals the angle of reflection.equal
- Write the answer. they are equal
Check. Both are measured from the normal. ✓
Method. Use the law of reflection.
- Recall. The angle of reflection equals the angle of incidence.equals
- Calculate. So the angle of reflection is 20 degrees.20
- Write the answer. 20
Check. The table shows 20 and 20. ✓
Method. Read the table.
- Read. The row for 50 degrees shows a reflection of 50 degrees.50
- Write the answer. 50
Check. Equal angles. ✓
Method. Recall the definition.
- Recall. A plane mirror is a flat reflecting surface.flat
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Plane means flat. ✓
Method. Recall the image.
- Recall. A plane mirror forms an image the same size as the object.same size
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. The image is the same distance behind the mirror. ✓
Method. Recall the definition.
- Recall. The normal is drawn at right angles to the surface.right angles
- Write the answer. at right angles
Check. Angles are measured from it. ✓
Method. Check the rule.
- Check. Both angles are measured from the normal.from the normal
- Decide. So Ruby is not right; the answer is no.no
- Write the answer. no
Check. Measure from the normal. ✓
Method. Use the normal.
- Think. The normal is at 90 degrees to the surface, so 90 minus 30 is the angle from the normal.90 - 30
- Calculate. So the angle of incidence is 60 degrees.90 - 30 = 60
- Write the answer. 60
Check. Both angles from the normal are 60 degrees. ✓
Method. Think about the light.
- Think. Light from behind is reflected by the mirror.reflected
- Conclude. Following the law of reflection, it reaches the eyes.reaches the eyes
- Write the answer. Accept any sentence that says the light is reflected by the mirror, following the law of reflection, into the eyes.
Check. The image is seen where the reflected rays appear to come from. ✓
Method. Think about the rays.
- Think. The reflected rays spread out as if they came from behind the mirror.as if they came
- Conclude. So the image is seen behind the mirror, as a virtual image.virtual
- Write the answer. Accept any sentence that says the reflected rays appear to diverge from a point behind the mirror, so the brain sees the image there.
Check. The brain traces rays back in straight lines. ✓
Refraction
Refraction is the change in direction of light when it passes from one medium into another at an angle. Light slows down when it enters a denser medium, such as glass or water, and it bends towards the normal. When it leaves, it speeds up and bends away from the normal.
Key ideas
- Refraction is the change in direction of light between two media.
- Light slows and bends towards the normal when it enters a denser medium.
- Light speeds up and bends away from the normal when it leaves.
- A pencil in water looks bent because of refraction.
Method. Recall the rule.
- Recall. Light entering a denser medium bends towards the normal.towards the normal
- Decide. So the light bends towards the normal.towards the normal
- Write the answer. towards the normal
Check. Glass is denser than air. ✓
Method. Recall the speed.
- Recall. Light slows down in a denser medium such as glass.slows down
- Decide. So the light slows down.slows
- Write the answer. slows
Check. Light speeds up again on leaving the glass. ✓
Method. Recall the effect.
- Recall. The bending of light at the boundary between water and air is refraction.refraction
- Name it. So the effect is refraction.refraction
- Write the answer. refraction
Check. The pencil is straight; the light bends. ✓
Watch out
Watch out: refraction happens at a boundary between two media. Light in one medium travels in a straight line.
Exam tip
Exam tip. Say “light slows down and bends towards the normal when it enters a denser medium”.
Try it (with your teacher, or a grown-up at home). Look at the refraction picture. Draw the normal, and show the ray bending towards it as it enters the glass. Do not shine a torch at any glass near anyone’s eyes.
Exercise 12D Refraction
Method. Recall the word.
- Recall. The change in direction at a boundary is refraction.refraction
- Write the answer. refraction
Check. Refraction is basic at this level. ✓
Method. Recall the speed.
- Recall. Light slows in a denser medium such as glass.slows
- Write the answer. slow down
Check. Glass is denser than air. ✓
Method. Recall the rule.
- Recall. Light bends towards the normal when it enters a denser medium.towards the normal
- Write the answer. towards the normal
Check. It bends away when it leaves. ✓
Method. Recall the rule.
- Recall. Light leaving a denser medium bends away from the normal.away from the normal
- Write the answer. away from the normal
Check. It speeds up on leaving. ✓
Method. Recall the fact.
- Recall. Light travels in straight lines in the same medium.straight lines
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. It bends only at a boundary. ✓
Method. Recall the word.
- Recall. Refraction makes the pencil look bent.refraction
- Write the answer. refraction
Check. The pencil is not really bent. ✓
Method. Recall the normal.
- Recall. The normal is at right angles to the surface.right angles
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Angles are measured from it. ✓
Method. Check the speed.
- Check. Light slows down when it enters a denser medium such as glass.slows down
- Decide. So Ali is not right; the answer is no.no
- Write the answer. no
Check. Light is slower in glass than in air. ✓
Method. Check the rule.
- Check. Entering a denser medium bends the ray towards the normal.towards the normal
- Decide. So the drawing is correct; the answer is yes.yes
- Write the answer. yes
Check. The ray bends towards the normal in glass. ✓
Method. Think about the medium.
- Think. Glass is denser than air.denser
- Conclude. So light travels more slowly through it.more slowly
- Write the answer. Accept any sentence that says glass is a denser medium, so light travels more slowly through it than through air.
Check. Light speed depends on the medium. ✓
Method. Think about the normal.
- Think. A ray along the normal is already at right angles to the surface.already
- Conclude. So it does not bend, although it still slows down.does not bend
- Write the answer. Accept any sentence that says a ray along the normal enters without changing direction, because it is already at right angles to the surface.
Check. Bending needs an angle to the normal. ✓
The electromagnetic spectrum
The electromagnetic spectrum is the family of electromagnetic waves. In order from the longest wavelength to the shortest, they are: radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All of them travel at the same speed in a vacuum, about 300 000 000 m/s.
Shorter wavelengths, such as ultraviolet, X-rays and gamma rays, carry more energy and can damage cells. Longer wavelengths, such as radio waves, are generally safer.
The table gives a use and a hazard for each type of wave.
| Type of wave | Use | Hazard |
|---|---|---|
| radio waves | broadcasting and communication | low hazard |
| microwaves | cooking food and satellite communication | heating body tissue |
| infrared | thermal imaging and remote controls | skin burns from strong heating |
| visible light | seeing | bright light can damage the eyes |
| ultraviolet | sunbeds and some security marking | skin damage and sunburn |
| X-rays | medical imaging | can damage cells |
| gamma rays | sterilising equipment and cancer treatment | can damage cells |
Key ideas
- The electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- All electromagnetic waves travel at the same speed in a vacuum.
- Shorter wavelengths carry more energy and can damage cells.
- Visible light is the only part of the spectrum that the eye detects.
Method. Use the order of the spectrum.
- Recall. The order from longest to shortest ends with gamma rays.gamma rays
- Name it. So gamma rays have the shortest wavelength.gamma rays
- Write the answer. gamma rays
Check. Gamma rays carry the most energy. ✓
Method. Recall the speed.
- Recall. All electromagnetic waves travel at about 300 000 000 m/s in a vacuum.300 000 000
- Write. So the speed is 300 000 000 m/s.300000000
- Write the answer. 300000000 m/s
Check. Three hundred million metres per second. ✓
Method. Read the table.
- Read. The table gives medical imaging as a use of X-rays.X-rays
- Name it. So the wave is an X-ray.X-rays
- Write the answer. X-rays
Check. X-rays pass through soft tissue more easily than bone. ✓
Watch out
Watch out: the speed of all electromagnetic waves is the same in a vacuum. Their wavelength and frequency are different, so their uses and hazards differ.
Exam tip
Exam tip. Give the order of the spectrum, and then a use and a hazard for one named wave.
Try it (with your teacher, or a grown-up at home). Look at the spectrum picture and the table. Name one wave that is used in cooking and one that is used to see in the dark. Do not look at the Sun or any laser; use the picture only.
Exercise 12E The electromagnetic spectrum
Method. Read the start of the spectrum.
- Read. Radio waves are at the long-wavelength end of the spectrum.long-wavelength end
- Name it. So the longest wavelength is radio waves.radio waves
- Write the answer. radio waves
Check. Radio waves are used for broadcasting. ✓
Method. Read the end of the spectrum.
- Read. Gamma rays are at the short-wavelength end.short-wavelength end
- Name it. So the shortest wavelength is gamma rays.gamma rays
- Write the answer. gamma rays
Check. Gamma rays carry the most energy. ✓
Method. Recall the part.
- Recall. The eye detects visible light.visible light
- Write the answer. visible light
Check. Visible light is in the middle of the spectrum. ✓
Method. Read the table.
- Read. Infrared is used in remote controls.remote controls
- Write the answer. infrared
Check. Infrared is just below visible light. ✓
Method. Read the table.
- Read. Microwaves are used for cooking food.cooking
- Write the answer. microwaves
Check. Microwaves heat the water in food. ✓
Method. Read the table.
- Read. Ultraviolet waves cause skin damage and sunburn.skin damage
- Write the answer. ultraviolet
Check. Ultraviolet is just beyond visible light. ✓
Method. Read the table.
- Read. Gamma rays are used to sterilise equipment.sterilising
- Write the answer. gamma rays
Check. Gamma rays can damage cells. ✓
Method. Recall the fact.
- Recall. All electromagnetic waves travel at the same speed in a vacuum.same speed
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Their wavelengths differ, not their speeds. ✓
Method. Recall the fact.
- Recall. All electromagnetic waves have the same speed in a vacuum.same speed
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Only the wavelength differs. ✓
Method. Check the speed.
- Check. All electromagnetic waves travel at the same speed in a vacuum.same speed
- Decide. So Ruby is not right; the answer is no.no
- Write the answer. no
Check. Radio and X-rays differ in wavelength. ✓
Method. Read the table.
- Read. The hazard listed for gamma rays is that they can damage cells.damage cells
- Write the answer. can damage cells
Check. That is why they are used with care. ✓
Method. Compare the energy.
- Think. Shorter wavelength means more energy per wave.more energy
- Conclude. So ultraviolet can damage cells, while radio waves carry much less energy.damage cells
- Write the answer. Accept any sentence that says ultraviolet has a shorter wavelength and more energy, so it can damage skin and cells, whereas radio waves carry less energy.
Check. Energy and wavelength are linked. ✓
Method. Think about wavelength.
- Think. The waves differ in wavelength and frequency, not in speed.not in speed
- Conclude. So they interact with matter in different ways, giving different uses.different ways
- Write the answer. Accept any sentence that says the speed is the same, but their wavelengths and frequencies differ, so they interact with matter in different ways.
Check. Speed is shared, wavelength is not. ✓
Chapter summary
| Skill | Method |
|---|---|
| Sound | Longitudinal wave; needs a medium; faster in solids than liquids than gases. |
| Hearing and pitch | Humans hear 20 Hz to 20 000 Hz; higher frequency gives higher pitch. |
| Reflection | Angle of incidence equals angle of reflection, both measured from the normal. |
| Refraction | Light slows and bends towards the normal in a denser medium. |
| EM spectrum | Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma; same speed in a vacuum. |
Can you…?
- I can describe sound as a longitudinal wave, and say why it needs a medium
- I can use the frequency range of hearing and link frequency to pitch
- I can apply the law of reflection to a ray diagram
- I can describe refraction in simple terms
- I can order the electromagnetic spectrum, and give a use and a hazard for each type
Key terms
- longitudinal wave
- a wave in which the particles vibrate parallel to the direction of travel
- compression
- a region where particles are crowded in a longitudinal wave
- rarefaction
- a region where particles are spread out in a longitudinal wave
- frequency
- the number of complete waves passing a point each second, in hertz
- normal
- a line drawn at right angles to a surface at the point of contact
- law of reflection
- the angle of incidence equals the angle of reflection
- refraction
- the change in direction of light when it passes between two media
- electromagnetic spectrum
- the family of electromagnetic waves, from radio to gamma rays
Exercise 12Z Check-up
Method. Recall the type.
- Recall. Sound is a longitudinal wave.longitudinal wave
- Write the answer. longitudinal wave
Check. Particles vibrate parallel to the travel. ✓
Method. Recall the fact.
- Recall. Sound needs a medium to carry its vibrations.medium
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Sound cannot cross a vacuum. ✓
Method. Recall the limit.
- Recall. Humans hear up to about 20 000 Hz.20 000
- Write the answer. 20000
Check. Above it is ultrasound. ✓
Method. Higher frequency means higher pitch.
- Decide. The higher frequency is 3000 Hz, so it has the higher pitch.3000
- Write the answer. 3000
Check. Frequency sets pitch. ✓
Method. Use the law of reflection.
- Recall. The angle of reflection equals the angle of incidence.equals
- Calculate. So the angle of reflection is 45 degrees.45
- Write the answer. 45
Check. The table shows equal angles. ✓
Method. Recall the word.
- Recall. The line at right angles to a surface is the normal.normal
- Write the answer. normal
Check. Angles are measured from it. ✓
Method. Recall the rule.
- Recall. Light entering a denser medium bends towards the normal.towards the normal
- Write the answer. towards the normal
Check. Glass is denser than air. ✓
Method. Read the spectrum.
- Read. Gamma rays are at the short end.short end
- Write the answer. gamma rays
Check. They carry the most energy. ✓
Method. Recall the speed.
- Recall. All electromagnetic waves travel at 300 000 000 m/s in a vacuum.300 000 000
- Write the answer. 300000000
Check. Three times ten to the eighth. ✓
Method. Think about the particles.
- Think. Particles in water are closer together than in air.closer together
- Conclude. So the vibration is passed on more quickly.more quickly
- Write the answer. Accept any sentence that says the particles in water are closer together, so they pass the vibration on more quickly than in air.
Check. Closer particles, faster vibrations. ✓
Total 13 marks. Allow about 13 minutes.
Exercise 12X Practice test questions
Method. Recall the type.
- Recall. Parallel vibration is a longitudinal wave.longitudinal wave
- Write the answer. longitudinal wave
Check. Sound is one example. ✓
Method. Compare with the range.
- Compare. Ten thousand is between 20 and 20 000.between
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. It is audible to humans. ✓
Method. Use the law of reflection.
- Recall. The angle of reflection equals the angle of incidence.equals
- Calculate. So the angle of reflection is 30 degrees.30
- Write the answer. 30
Check. Both measured from the normal. ✓
Method. Think about refraction.
- Think. Light slows and bends when it passes from water into air.bends
- Conclude. So the pencil appears in a different place and looks bent.looks bent
- Write the answer. Accept any sentence that says light is refracted at the surface of the water, so the light from the pencil changes direction and the pencil looks bent.
Check. The pencil is actually straight. ✓
Method. Read the table.
- Read. Microwaves are used for cooking.cooking
- Write the answer. microwaves
Check. Microwaves heat water in food. ✓
Method. Recall the speed.
- Recall. The speed is 300 000 000 m/s in a vacuum.300 000 000
- Write the answer. 300000000
Check. Same for all types. ✓
Method. Read the table.
- Read. Ultraviolet causes skin damage and sunburn.skin damage
- Write the answer. ultraviolet
Check. Ultraviolet is just beyond the visible range. ✓
Method. Think about the reference.
- Think. A reference line is needed for the angles.reference line
- Conclude. The normal is that reference, so the rule is consistent.consistent
- Write the answer. Accept any sentence that says the normal is the reference line, so the angles are measured from it for a consistent rule of reflection.
Check. The normal is the standard reference. ✓
Method. Read the spectrum.
- Read. Radio waves are at the long-wavelength end.long-wavelength end
- Write the answer. radio waves
Check. Radio waves are used for broadcasting. ✓
Method. Check the speed.
- Check. All electromagnetic waves travel at the same speed in a vacuum.same speed
- Decide. So Ali is not right; the answer is no.no
- Write the answer. no
Check. Only the wavelength differs. ✓