Wave properties

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Notes de leçon

Big idea: waves carry energy

Big idea. Key concept: Systems. A wave system has a source that vibrates, a medium the vibration passes through and a receiver that detects it, such as an ear or an eye.
Related concept: Models. We cannot see sound or light travelling, so scientists use models such as a rope, a spring and a ripple tank to picture how waves behave.
Global context: Scientific and technical innovation. Mobile phones, Wi-Fi, ultrasound scans, musical instruments and satellites all depend on our understanding of waves.
A wave is a vibration that transfers energy from one place to another without transferring matter.
When a wave crosses a pond, the water moves up and down on the spot. A floating duck bobs up and down but is not carried across the pond.
The energy is carried along by the wave, which is why waves can make things move or heat things up when they arrive.

A duck bobbing as waves pass

Transverse and longitudinal waves

In a transverse wave the vibrations are at right angles to the direction in which the wave travels. A wave on a rope, ripples on water and all light waves are transverse.
The highest points of a transverse wave are crests and the lowest points are troughs.
In a longitudinal wave the vibrations are parallel to the direction of travel. Push and pull a spring along its length and a longitudinal wave moves along it.
A longitudinal wave is made of compressions, where the coils are squashed together, and rarefactions, where they are spread out.
Sound is a longitudinal wave. It must have a medium, such as air, water or a solid, to travel through, so it cannot travel through a vacuum.
Light and the other electromagnetic waves are transverse and can travel through a vacuum, which is how sunlight reaches us across space.

A transverse wave on a rope

Describing a wave: amplitude and wavelength

The rest position (or undisturbed position) is where the medium would be if there were no wave.
The amplitude is the maximum distance a point on the wave moves from its rest position. It is measured from the rest position to a crest (or to a trough), not from trough to crest.
If a wave rises 3 cm above the rest position and falls 3 cm below it, its amplitude is 3 cm, not 6 cm.
The wavelength, symbol λ, is the distance from one point on a wave to the same point on the next wave, for example from crest to crest. It is measured in metres (m).
In a longitudinal wave, wavelength is the distance from one compression to the next compression.
A larger amplitude means the wave carries more energy: a louder sound, a brighter light, a bigger water wave.

Amplitude and wavelength of a wave

Frequency, period and wave speed

The frequency is the number of complete waves that pass a point each second. It is measured in hertz (Hz). 1 Hz is one wave per second.
The period, T, is the time for one complete wave to pass. It is measured in seconds (s). Period = 1 / frequency, so a 4 Hz wave has a period of 1 / 4 = 0.25 s.
The wave speed, v, is how far a wave travels each second, measured in metres per second (m/s).
Wave speed = frequency x wavelength (v = f x λ). It can be rearranged to f = v / λ and λ = v / f.
Worked example. A water wave has a frequency of 5 Hz and a wavelength of 0.30 m. Wave speed = 5 Hz x 0.30 m = 1.5 m/s.
In one material the wave speed stays the same, so a higher frequency means a shorter wavelength. You can see this in a ripple tank when the dipper vibrates faster.

Low and high frequency water waves

Reflection, refraction and the electromagnetic spectrum

Reflection is when a wave bounces off a surface. The angle of incidence equals the angle of reflection, with both angles measured from the normal, a line at 90° to the surface. An echo is a reflected sound wave.
Refraction is the change in direction of a wave when it passes into a material where its speed changes. Water waves slow down and their wavelength shortens when they move from deep water into shallow water.
The frequency of a wave does not change when it refracts. Only the speed and wavelength change.
The electromagnetic spectrum is a family of transverse waves. In order of decreasing wavelength they are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
All electromagnetic waves travel at the same speed in a vacuum, about 300 000 000 m/s. They differ in wavelength and frequency.
Uses include radio and TV (radio waves), cooking and phone signals (microwaves), remote controls (infrared), seeing (visible light) and hospital imaging (X-rays). High-frequency waves such as ultraviolet, X-rays and gamma rays can damage living cells.

Angles of incidence and reflection

Think like a scientist: how does frequency affect wavelength?

Question. In a ripple tank, what happens to the wavelength of the water waves when the frequency of the vibrating dipper is increased?
The independent variable is the frequency of the dipper. The dependent variable is the wavelength of the waves, measured in centimetres.
To make it a fair test, keep the depth of the water, the size of the tank and the shape of the dipper the same. Use only the low-voltage motor supplied by your teacher.
To measure wavelength more accurately, measure the distance across several waves (for example 5) on a ruler and divide by the number of waves. This reduces the effect of a small reading error.
Repeat the measurement for each frequency and calculate a mean.
Inquiry task. Plan an investigation into whether the depth of the water changes the speed of the waves. State the variable you change, the variable you measure, three variables you control, and how you would reduce measurement error.

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1.What does a wave transfer from one place to another?
Easy
- AMatter
- BEnergy
- CParticles of the medium
- DMass and energy equally
2.A duck floats on a pond while waves pass underneath it. What happens to the duck?
Easy
- AIt is carried across the pond at the speed of the waves
- BIt spins around on the spot without any up and down motion
- CIt sinks a little lower after each wave passes
- DIt bobs up and down and stays in about the same place
3.In a transverse wave, how do the vibrations move compared with the direction the wave travels?
Easy
- AAt right angles to it
- BIn the same direction as it
- CBackwards and forwards along it
- DOnly in the direction opposite to it
4.Which of these is a longitudinal wave?
Easy
- ALight
- BRadio waves
- CSound in air
- DA wave on a rope
5.What is the amplitude of a wave?
Easy
- AThe distance between two crests next to each other
- BThe number of waves that pass a point each second
- CThe maximum distance of a point from its rest position
- DThe time taken for one complete wave to pass
6.What is the wavelength of a transverse wave?
Easy
- AThe distance from one crest to the next crest
- BThe distance from a crest down to the next trough
- CThe distance from the rest position up to a crest
- DThe number of crests passing a point each second
7.In which unit is frequency measured?
Easy
- AMetres (m)
- BSeconds (s)
- CMetres per second (m/s)
- DHertz (Hz)
8.Which equation correctly gives wave speed?
Easy
- AWave speed = frequency / wavelength
- BWave speed = frequency x wavelength
- CWave speed = wavelength / frequency
- DWave speed = frequency + wavelength

Plus de questions

9.Which rearrangement of the wave speed equation gives the wavelength?
Medium
- AWavelength = wave speed x frequency
- BWavelength = wave speed / frequency
- CWavelength = frequency / wave speed
- DWavelength = wave speed + frequency
10.The amplitude of a wave is doubled but its frequency stays the same. What changes?
Medium
- AThe wave travels twice as fast
- BThe wavelength becomes half as long
- CThe frequency becomes twice as large
- DThe wave carries more energy
11.In a ripple tank the water depth stays the same, so the wave speed is constant. The frequency of the dipper is increased. What happens to the wavelength?
Medium
- AIt decreases
- BIt increases
- CIt stays the same
- DIt becomes zero
12.What happens when a water wave moves from deep water into shallow water?
Medium
- AThe wavelength gets longer because the wave speeds up
- BThe frequency changes but the wavelength is unchanged
- CThe wavelength gets shorter because the wave slows down
- DThe wave stops and all its energy disappears
13.What is refraction?
Easy
- AA wave bouncing off a surface at the same angle
- BA wave being taken in by a material and heating it
- CA change in the direction of a wave when its speed changes in a new material
- DA wave spreading out after it passes through a gap
14.A ray of light hits a flat mirror with an angle of incidence of 30° to the normal. What is the angle of reflection?
Medium
- A30°
- B60°
- C15°
- D90°
15.Which statement is true of all electromagnetic waves?
Medium
- AThey are all longitudinal waves
- BThey all have the same wavelength
- CThey need air in order to travel
- DThey can travel through a vacuum
16.Which part of the electromagnetic spectrum has the shortest wavelength?
Easy
- ARadio waves
- BGamma rays
- CMicrowaves
- DVisible light
17.Wave A and wave B travel at the same speed in the same material. Wave A has the higher frequency. Which statement is correct?
Medium
- AWave A has the longer wavelength
- BWave A has the shorter wavelength
- CThe two waves have the same wavelength
- DWave A has the larger amplitude
18.Two sound waves travel through air at the same speed. One has a frequency of 200 Hz and the other has 400 Hz. How do their wavelengths compare?
Hard
- AThe 400 Hz wave has twice the wavelength of the 200 Hz wave
- BThe two waves have exactly the same wavelength
- CThe 400 Hz wave has four times the wavelength of the other
- DThe 400 Hz wave has half the wavelength of the 200 Hz wave
19.A student investigates how the frequency of a ripple tank dipper affects the wavelength of the waves. What is the dependent variable?
Easy
- AThe wavelength of the waves
- BThe frequency of the dipper
- CThe depth of the water
- DThe size of the tank
20.In the same investigation, which variable should be kept the same to make it a fair test?
Medium
- AThe frequency of the dipper
- BThe wavelength of the waves
- CThe depth of the water
- DThe number of waves counted
21.A student measures the wavelength of ripples by measuring one wavelength with a ruler. Which change would improve the method most?
Hard
- AUse a ruler with thicker markings on its edge
- BMeasure the same single wavelength a second time
- CMeasure across 5 waves and divide the distance by 5
- DMeasure the crest nearest the dipper only
22.Why can light travel from the Sun to Earth when sound cannot travel through the vacuum of space?
Hard
- ALight is an electromagnetic wave that needs no medium, but sound needs particles to vibrate
- BLight is a longitudinal wave, which needs no particles
- CSound is a transverse wave, which cannot cross a gap
- DLight is much louder than sound, so it travels further
23.Which of these are transverse waves? (select all that apply)
Medium
- ASound in air
- BLight
- CA wave that compresses a spring along its length
- DRipples on water
- EA wave on a rope
24.Which statements describe a longitudinal wave? (select all that apply)
Medium
- AIt is made of compressions and rarefactions
- BIt has crests and troughs
- CThe vibrations are parallel to the direction of travel
- DIt can travel through a vacuum
- ESound is an example
25.A water wave moves from deep water into shallow water. Which statements are true? (select all that apply)
Hard
- AThe frequency stays the same
- BThe wavelength increases
- CThe wave speed decreases
- DThe wavelength decreases
- EThe frequency increases
26.A wave transfers energy without transferring matter.
Easy
True or false?
27.The amplitude of a wave is the distance from one crest to the next crest.
Easy
True or false?
28.Sound can travel through a vacuum.
Easy
True or false?
29.Light is a transverse wave.
Easy
True or false?
30.In the same material, a wave with a higher frequency always travels faster than a wave with a lower frequency.
Medium
True or false?
31.A wave rises 3 cm above its rest position and falls 3 cm below it, so its amplitude is 6 cm.
Hard
True or false?
32.Match each term to its meaning.
Easy
- Crest
- Trough
- Amplitude
- Wavelength
- The highest point of a transverse wave
- The lowest point of a transverse wave
- The maximum distance from the rest position
- The distance from one crest to the next
33.Match each wave quantity to its unit.
Medium
- Frequency
- Wavelength
- Wave speed
- Period
- Hertz (Hz)
- Metres (m)
- Metres per second (m/s)
- Seconds (s)
34.Match each type of electromagnetic wave to a use.
Easy
- Radio waves
- Microwaves
- Infrared
- X-rays
- Broadcasting TV and radio
- Cooking food and carrying phone signals
- Remote controls for televisions
- Taking pictures of bones in hospitals
35.Put these electromagnetic waves in order from the longest wavelength to the shortest wavelength.
Medium
- Visible light
- X-rays
- Radio waves
- Gamma rays
- Infrared
- Ultraviolet
- Microwaves
36.Four waves all travel at 6 m/s in the same material. Their frequencies are: P 2 Hz, Q 6 Hz, R 3 Hz, S 1 Hz. Put them in order from the shortest wavelength to the longest.
Hard
- Wave P
- Wave Q
- Wave S
- Wave R
37.Put these steps in order to measure the frequency of the waves made by a dipper in a ripple tank.
Medium
- Divide the number of waves by the time in seconds
- Start a stopwatch and begin counting waves passing a mark
- Record the frequency in hertz
- Stop the stopwatch after a set time
38.A water wave has a frequency of 5 Hz and a wavelength of 0.30 m. Using wave speed = frequency x wavelength, what is the wave speed in metres per second?
Easy
39.Sound travels through air at about 340 m/s. A sound has a frequency of 50 Hz. Using wavelength = wave speed / frequency, what is its wavelength in metres?
Medium
40.A wave has a frequency of 4 Hz. Using period = 1 / frequency, what is its period in seconds?
Easy
41.One complete wave takes 0.02 s to pass a point. Using frequency = 1 / period, what is the frequency in hertz?
Medium
42.In a ripple tank, 12 waves pass a mark in 4 s, and the wavelength is 0.02 m. Work out the frequency in hertz, then use wave speed = frequency x wavelength to find the wave speed in metres per second.
Hard
43.
These readings come from one experiment on a wave on a rope. The displacement is measured from the rest position.
Distance along the wave (m) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Displacement from rest position (cm) 0 3 4 3 0 -3 -4 -3 0
What is the amplitude of the wave in centimetres?
Medium
44.
The wave in this table repeats its pattern after 0.8 m, and a student counts a frequency of 2.5 Hz.
Distance along the wave (m) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Displacement from rest position (cm) 0 3 4 3 0 -3 -4 -3 0
Use the wavelength from the table and wave speed = frequency x wavelength to find the wave speed in metres per second.
Hard
45.In a thunderstorm, the thunder is heard 3.0 s after the lightning flash. Sound travels at about 340 m/s in air, and the light arrives almost instantly. Using distance = speed x time, how far away is the lightning in metres?
Hard
46.A radio wave has a frequency of 100 000 000 Hz. All electromagnetic waves travel at 300 000 000 m/s in air. Using wavelength = wave speed / frequency, what is the wavelength of the radio wave in metres?
Hard
47.Complete the sentence about transverse waves.
Easy
In a ____ wave the vibrations are at right angles to the direction of travel.
48.Complete the wave speed equation.
Easy
Wave speed = frequency x ____.
49.Complete the sentence about period.
Medium
The time taken for one complete wave to pass a point is called the ____.
50.Describe one difference between a transverse wave and a longitudinal wave, and give one example of each.
Medium
51.In a ripple tank, the dipper is made to vibrate faster and the waves become closer together. Explain why the wavelength decreases.
Hard
52.Two students are 100 m apart. One claps and the other starts a stopwatch when they see the clap and stops it when they hear it, to find the speed of sound. Evaluate this method and suggest one improvement.
Hard

Notes de révision

## Wave properties - A **wave** transfers **energy** without transferring matter. - **Transverse** waves vibrate at right angles to the direction of travel; **longitudinal** waves vibrate parallel to it. - **Amplitude** is measured from the rest position; **wavelength** is crest to crest. - **Frequency** (Hz) is waves per second; **period = 1 / frequency**. - **Wave speed = frequency x wavelength.** - In one material, higher frequency means shorter wavelength. - Reflection: angle of incidence = angle of reflection. Refraction: direction changes when speed changes. - The **electromagnetic spectrum** is transverse, travels at the same speed in a vacuum and runs from radio waves to gamma rays.

Wave: A vibration that transfers energy from one place to another without transferring matter.
Transverse wave: A wave whose vibrations are at right angles to the direction of travel.
Longitudinal wave: A wave whose vibrations are parallel to the direction of travel, made of compressions and rarefactions.
Crest: The highest point of a transverse wave.
Trough: The lowest point of a transverse wave.
Amplitude: The maximum distance a point on a wave moves from its rest position.
Wavelength: The distance from one point on a wave to the same point on the next wave, in metres.
Frequency: The number of complete waves passing a point each second, in hertz (Hz).
Period: The time for one complete wave to pass a point, in seconds; period = 1 / frequency.
Wave speed: The distance a wave travels each second, in m/s; wave speed = frequency x wavelength.
Reflection: A wave bouncing off a surface, with angle of incidence equal to angle of reflection.
Refraction: A change in the direction of a wave when its speed changes in a new material.
Normal: A line drawn at 90 degrees to a surface, from which angles are measured.
Electromagnetic spectrum: The family of transverse waves from radio waves to gamma rays that travel at the same speed in a vacuum.

Cartes mémo

What does a wave transfer?

Energy, but not matter.

What is a transverse wave?

A wave whose vibrations are at right angles to the direction of travel.

What is a longitudinal wave?

A wave whose vibrations are parallel to the direction of travel.

Give an example of a longitudinal wave.

Sound.

What is the amplitude of a wave?

The maximum distance from the rest position to a crest or trough.

What is wavelength?

The distance from one crest to the next crest, in metres.

What is frequency and its unit?

The number of waves per second, measured in hertz (Hz).

How do you find the period of a wave?

Period = 1 / frequency, in seconds.

Write the wave speed equation.

Wave speed = frequency x wavelength.

A wave has f = 5 Hz and wavelength 0.30 m. What is its speed?

1.5 m/s.

What happens to wavelength if frequency rises and speed stays constant?

The wavelength gets shorter.

What is reflection?

A wave bouncing off a surface. The angle of incidence equals the angle of reflection.

What is refraction?

A change in direction when a wave's speed changes in a new material.

What happens to water waves in shallow water?

They slow down and their wavelength shortens.

Which electromagnetic wave has the shortest wavelength?

Gamma rays.

Why can light cross space but sound cannot?

Light needs no medium; sound needs particles to vibrate.

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