Physics · 3 Waves

3.1 General properties of waves

Before you start

Speed is distance travelled per unit time, in m/s.
Converting centimetres to metres: divide by 100.
Rearranging an equation with three quantities, such as $a = b c$ into $b = a / c$ .
Entering numbers in standard form on a calculator, such as $3.0 \times 10^{8}$ .
Energy can be transferred from one place to another.

Syllabus objectives covered (10)

Know that waves transfer energy without transferring matter
Describe what is meant by wave motion as illustrated by vibrations in ropes and springs, and by experiments using water waves
Describe the features of a wave in terms of wavefront, wavelength, frequency, crest (peak), trough, amplitude and wave speed
Recall and use the equation for wave speed v = f λ
Know that for a transverse wave, the direction of vibration is at right angles to the direction of propagation and understand that electromagnetic radiation, water waves and seismic S-waves (secondary) can be modelled as transverse
Know that for a longitudinal wave, the direction of vibration is parallel to the direction of propagation and understand that sound waves and seismic P-waves (primary) can be modelled as longitudinal
Describe how waves can undergo: (a) reflection at a plane surface (b) refraction due to a change of speed (c) diffraction through a narrow gap
Describe the use of a ripple tank to show: (a) reflection at a plane surface (b) refraction due to a change in speed caused by a change in depth (c) diffraction due to a gap (d) diffraction due to an edge
Describe how wavelength and gap size affects diffraction through a gap
Describe how wavelength affects diffraction at an edge

Part 1 of 7

What a wave actually carries

A wave is a disturbance that travels. Tie one end of a long rope to a post and flick the other end up and down once: a hump runs along the rope to the post. The rope itself stays where it was.
That is the first idea of the whole topic: waves transfer energy without transferring matter. The energy you put in at your end reaches the post. The rope does not.
You can prove the rope does not travel. Tie a ribbon to the middle of it. As the wave passes, the ribbon moves up and then down and settles back where it started. It never moves along the rope towards the post.
Here is what is happening. Each small piece of rope vibrates about its own fixed position, and as it moves it pulls on the piece next to it. So the vibration is handed on, piece to piece. The pattern travels; the material only vibrates. That is what wave motion means.
A long spring (a slinky) lying on a bench shows the same thing. Move one end from side to side and a wave runs along it, with each coil vibrating across the spring.
The spring can carry a second kind of wave. Push and pull the end along the spring's own length, and regions of bunched-up coils run along it. Each coil moves back and forth along the spring, and again returns to where it started.
Water waves show it too. Drop a stone into a still pond and circular ripples spread outwards. A cork floating on the surface bobs up and down as they pass, but it is not carried outwards with them. The ripples carry energy across the pond, not the water.
In the laboratory, water waves are studied in a ripple tank: a shallow tray of water in which a vibrating bar makes a steady train of waves. It is the apparatus for the reflection, refraction and diffraction experiments later in this lesson.

Part 2 of 7

The words that describe a wave

Picture a snapshot of the rope at one instant. The horizontal line through the middle of the wave is the rest position: where the rope would lie if there were no wave on it.
graph diagram — not drawn yet
A transverse wave drawn as a smooth sine curve of two and a half cycles against a horizontal axis labelled distance, with a dashed rest-position line through the middle; label one crest and one trough, a vertical double arrow from the rest line to a crest labelled amplitude, and a horizontal double arrow from one crest to the next crest labelled wavelength λ.
Amplitude starts at the rest line, not at a trough. Wavelength runs from crest to the next crest.
A crest (or peak) is a highest point of the wave above the rest position. A trough is a lowest point below it.
The amplitude is the maximum displacement from the rest position: from the rest line up to a crest, or down to a trough. The trap: it is not the distance from a trough to a crest. That distance is twice the amplitude.
The wavelength, $λ$ (the Greek letter lambda), is the distance from a point on one wave to the same point on the next wave: crest to the next crest, or trough to the next trough. It is a length, so it is measured in metres. Crest to the next trough is only half a wavelength.
The frequency, $f$ , is the number of complete waves passing a point each second. That is the same as the number of complete vibrations the source makes each second. Its unit is the hertz (Hz), and $1$ Hz means one wave per second.
The wave speed, $v$ , is the distance the wave travels each second, measured in m/s.
A wavefront is a line joining points on a wave that are at the same stage of their vibration. The simplest example is a line along a crest. Look down on water waves and every crest you can see is a wavefront.
Two facts let you read any wavefront diagram. First, the distance between neighbouring wavefronts is one wavelength. Second, the wave travels at right angles to its wavefronts.
The trap with counting: when you measure across several wavefronts, the number of wavelengths is one fewer than the number of wavefronts. Six crests have five gaps between them, so the distance from the first crest to the sixth is five wavelengths.

Part 3 of 7

Wave speed: $v = f λ$

Paper 4 gives you no formula sheet, so you must recall this equation. It is easier to keep once you see where it comes from.
Suppose a source makes $f$ waves every second, and each one is $λ$ long. In one second, $f$ whole waves leave the source, laid end to end, so the front of the wave has moved $f$ lots of $λ$ . Distance moved in one second is the speed, so:
$v = f λ$
Units: $v$ in m/s, $f$ in Hz and $λ$ in m. If the wavelength is given in cm, convert it to metres first. Hz multiplied by cm gives cm/s, and if you then write m/s your answer is $100$ times too big.
To find the frequency or the wavelength, rearrange. Divide both sides by $λ$ to get $f$ on its own; divide both sides by $f$ to get $λ$ on its own.
$f = \frac{v}{λ} λ = \frac{v}{f}$
The frequency is set by the source: the hand on the rope, or the vibrating bar in the ripple tank. Reflection, refraction and diffraction never change the frequency of a wave.
This matters when a wave passes into a different medium. Its speed changes, and its frequency does not, so its wavelength must change in the same proportion as the speed. Slower means shorter waves.
For radio waves and light the numbers are very large or very small, so questions give them in standard form, such as $3.0 \times 10^{8}$ m/s. Type them with your calculator's standard-form key rather than counting zeros.

Part 4 of 7

Transverse and longitudinal waves

Every wave has two directions you can compare: the direction in which the material vibrates, and the direction in which the wave travels (its direction of propagation). The two kinds of wave are defined by how these two directions are related.
apparatus diagram — not drawn yet
Two horizontal slinky springs, one above the other, each with an arrow along its length labelled direction of wave travel: the upper spring displaced sideways into a wave shape with a short double arrow at right angles to the spring labelled vibration; the lower spring straight but with coils bunched in places labelled compression and spread in places labelled rarefaction, with a short double arrow along the spring labelled vibration.
Same spring, two waves. Compare the vibration arrow with the travel arrow.
In a transverse wave, the direction of vibration is at right angles to the direction of propagation. The rope is the model: the rope moves up and down while the wave moves along it.
Electromagnetic radiation (light, radio waves and the rest of the electromagnetic spectrum), water waves and seismic S-waves (secondary waves from an earthquake) can all be modelled as transverse.
Notice the syllabus words 'modelled as'. A transverse wave is a picture that describes how these waves behave, and it is the picture you use in your answers. Light can even travel through a vacuum, where there is no material to vibrate, and the transverse model still describes it.
In a longitudinal wave, the direction of vibration is parallel to the direction of propagation. The spring pushed and pulled along its length is the model: each coil moves back and forth along the same line that the wave travels along.
In a longitudinal wave the coils bunch together in some places, called compressions, and spread apart in others, called rarefactions. These are what travel along. One wavelength is the distance from the centre of one compression to the centre of the next.
Sound waves and seismic P-waves (primary waves from an earthquake) can be modelled as longitudinal. In sound travelling through air, the air particles vibrate back and forth along the direction the sound travels.
A memory aid for the earthquake waves: P for push–pull (longitudinal), S for side-to-side shaking (transverse). It is only an aid, not a definition.
When you define either kind, name both directions: 'the direction of vibration' and 'the direction of propagation' (or 'the direction the wave travels'). 'It goes up and down' describes the rope, not a definition of a transverse wave.

Part 5 of 7

Reflection, refraction and diffraction

A wave can do three things when it meets something. For each one, learn what happens and which of speed, wavelength and frequency change.
Reflection happens when a wave hits a plane (flat) surface and bounces back. Angles are measured from the normal, a line at right angles to the surface, and the angle of incidence equals the angle of reflection.
A reflected wave is still in the same medium, so its speed, wavelength and frequency are all unchanged. In a sketch, the reflected wavefronts have the same spacing as the incoming ones.
Refraction is a change in the direction of a wave caused by a change in its speed, when the wave passes into a different medium.
Why does a change of speed change the direction? Picture a straight wavefront reaching the boundary at an angle. One end of it enters the slow medium first and slows down while the other end is still moving fast. The fast end gains on the slow end, so the wavefront swings round and the wave heads off in a new direction.
A wave that slows down bends towards the normal. A wave that meets the boundary head-on, travelling along the normal, still changes speed and wavelength, but its direction does not change, because the whole wavefront slows at the same moment.
In refraction the frequency is unchanged, because the source has not changed. So by $v = f λ$ , when the speed falls the wavelength falls in the same proportion, and the wavefronts are drawn closer together.
Diffraction is the spreading out of a wave as it passes through a gap or past the edge of an obstacle. Waves passing through a narrow gap fan out into the region beyond it instead of carrying straight on.
A diffracted wave is still in the same medium, so its speed, wavelength and frequency are unchanged. The trap in a sketch: the curved wavefronts beyond the gap must have the same spacing as the straight ones arriving at it.

Part 6 of 7

The ripple tank: seeing all three

A ripple tank is a shallow tray of water with a transparent bottom. A bar driven by a small motor dips in and out of the water and makes straight (plane) waves at a steady frequency.
apparatus diagram — not drawn yet
Side view of a ripple tank: a shallow transparent-bottomed tray of water on legs, a lamp above it, a white screen on the floor beneath it, and a straight bar touching the water surface at one end attached to a small motor; labels lamp, water, vibrating bar, motor, screen.
Light from the lamp passes through the water, and each crest shows as a bright line on the screen.
A lamp shines through the water onto a screen. Each crest acts like a lens and focuses the light into a bright line, so the wavefronts appear on the screen as a pattern of bright and dark lines.
The bright lines are crests, so the distance between neighbouring bright lines is one wavelength. Freezing the pattern, with a stroboscope or a photograph, lets you measure across several wavelengths at once and divide.
(a) Reflection at a plane surface. Stand a straight barrier in the water at an angle to the waves. The wavefronts bounce off it and leave at the same angle to the barrier as they arrived, with their spacing unchanged.
ray diagram — not drawn yet
Four top-view panels of straight wavefronts with arrows showing direction of travel: (a) wavefronts striking a plane barrier at an angle and reflecting at an equal angle with equal spacing; (b) wavefronts crossing at an angle onto a region labelled shallow water, where they are closer together and travelling in a new direction bent towards the normal; (c) wavefronts passing through a gap about one wavelength wide between two barriers and emerging as near-semicircular arcs with unchanged spacing; (d) wavefronts passing the end of a single barrier and curving round into the region behind it with unchanged spacing.
Spacing stays the same in (a), (c) and (d). Only in (b), where the speed changes, do the wavefronts close up.
(b) Refraction caused by a change in depth. Lay a glass or plastic plate on the bottom of part of the tank, so the water above it is shallower. Waves travel more slowly in shallower water.
Over the plate, the wavefronts crowd closer together, because the speed has fallen and the frequency has not. If the edge of the plate is at an angle to the wavefronts, the waves also change direction as they cross onto it.
The tank makes the cause clear. Nothing about the water changes except its depth, so the change in speed caused by the change in depth must be what changes the direction.
(c) Diffraction due to a gap. Put two barriers in a line with a gap between them. The waves that pass through the gap spread out into the region beyond, as curved wavefronts.
(d) Diffraction due to an edge. Use a single barrier. The waves passing its end curve round into the region behind it, which would be a calm 'shadow' if waves only travelled in straight lines.
Whichever one you describe, name the apparatus that causes the effect (barrier, shallow plate, gap, edge) and say what the wavefronts do afterwards, including whether their spacing changes.

Part 7 of 7

How much does a wave diffract?

Diffraction happens at every gap, but how much the wave spreads depends on two lengths: the width of the gap and the wavelength of the wave. What matters is how they compare.
ray diagram — not drawn yet
Two top-view panels of equally spaced straight wavefronts arriving at a barrier: left, a gap many wavelengths wide, with the wavefronts beyond it staying straight and curving only slightly at their ends; right, a gap about one wavelength wide, with the wavefronts beyond it spreading as near-semicircular arcs; the spacing is the same before and after the gap in both panels.
Same wavelength, different gaps. The gap closer in size to the wavelength gives far more spreading.
When the gap is much wider than the wavelength, most of the wave carries straight on. The wavefronts beyond the gap stay straight and curve only slightly at their ends.
When the gap is about the same size as the wavelength, the wave spreads out strongly. The wavefronts beyond the gap become almost semicircles.
So, for a fixed wavelength, narrowing a wide gap down towards the size of one wavelength increases the diffraction. For a gap of fixed width, a longer wavelength diffracts more, because it is closer in size to the gap.
Write it as a comparison. 'There is more diffraction because the gap is closer in size to the wavelength' is a complete answer. 'The gap is small' is not, because it does not say small compared with what.
At an edge there is no gap width, so only the wavelength matters. Longer-wavelength waves bend further round the edge into the region behind the barrier. Shorter-wavelength waves bend round it less.
This links back to $v = f λ$ . In the ripple tank, lowering the frequency of the vibrating bar makes the wavelength longer, so the waves diffract more through the same gap and round the same edge.
Diffraction helps explain why you can hear someone talking in the next room through an open door without seeing them. Many sound wavelengths are similar in size to a doorway, so sound diffracts strongly through it. The wavelength of light is far smaller than a doorway, so its diffraction there is too small to notice.

Worked examples

Example 1. A student freezes the wave pattern in a ripple tank and measures the distance from the first crest to the sixth crest as $12$ cm. She counts $20$ waves passing a mark in $4.0$ s. Calculate (a) the wavelength, (b) the frequency and (c) the speed of the waves.

Show the working

(a) Count gaps, not crests. From the first crest to the sixth there are five gaps, so $12$ cm is five wavelengths.
$λ = \frac{12}{5} = 2.4 cm = 0.024 m$
Convert to metres now, because the speed must come out in m/s. Divide by $100$ : $2.4$ cm is $0.024$ m.
(b) Frequency is the number of waves per second. $20$ waves in $4.0$ s is $20 \div 4.0$ .
$f = \frac{20}{4.0} = 5.0 Hz$
(c) Recall $v = f λ$ and substitute the frequency and the wavelength in metres.
$v = 5.0 \times 0.024 = 0.12 m/s$
Check the units. $0.12$ m/s is $12$ cm each second. Had you multiplied $5.0$ by $2.4$ cm and written m/s, you would have claimed $12$ m/s, a speed $100$ times too big.

Answer: (a) $0.024$ m ( $2.4$ cm); (b) $5.0$ Hz; (c) $0.12$ m/s

Example 2. Water waves in the deep part of a ripple tank travel at $0.18$ m/s and have a wavelength of $3.0$ cm. They pass into a shallow region, where their speed is $0.12$ m/s. Calculate the wavelength of the waves in the shallow region.

Show the working

You know the speed in the shallow region but not the frequency. The frequency is the same in both regions, because it is set by the vibrating bar, so find it from the deep-water values first.
Convert the wavelength to metres: $3.0$ cm is $0.030$ m. Then rearrange $v = f λ$ by dividing both sides by $λ$ .
$f = \frac{v}{λ} = \frac{0.18}{0.030} = 6.0 Hz$
Now use the same frequency with the new speed. Rearrange $v = f λ$ by dividing both sides by $f$ .
$λ = \frac{v}{f} = \frac{0.12}{6.0} = 0.020 m$
Check with proportion: the speed has fallen to $\frac{0.12}{0.18} = \frac{2}{3}$ of its value, so the wavelength must fall to $\frac{2}{3}$ of $3.0$ cm, which is $2.0$ cm. The two methods agree.

Answer: $0.020$ m ( $2.0$ cm)

Example 3. Straight water waves of wavelength $1.5$ cm pass through a gap $15$ cm wide in a ripple tank. Predict how the waves beyond the gap change, and explain why, when (a) the gap is narrowed to $1.5$ cm; (b) the gap is left at $15$ cm and the frequency of the vibrating bar is lowered.

Show the working

Start by comparing the two lengths. At first the gap is $15 \div 1.5 = 10$ wavelengths wide, so it is much wider than the wavelength. The waves go mostly straight through, curving only slightly at their ends.
(a) Now the gap is $1.5$ cm, the same size as the wavelength. That is the condition for strong spreading, so the waves beyond the gap spread out as nearly semicircular wavefronts.
Say what does not change: the water depth is the same, so the speed is the same, and the wavelength beyond the gap is still $1.5$ cm.
(b) Lowering the frequency makes each wave longer: $λ = v / f$ , and a smaller $f$ gives a larger $λ$ in the same water.
A longer wavelength is closer in size to the $15$ cm gap, so the waves diffract more: the wavefronts beyond the gap curve further round at their ends. The same reasoning applies at an edge, where longer waves bend further into the region behind the barrier.

Answer: (a) The waves spread out much more, as nearly semicircular wavefronts, because the gap is now about the same size as the wavelength; the wavelength stays $1.5$ cm. (b) The wavelength increases, so it is closer in size to the gap and the waves diffract more.

Where people go wrong

Mistake: Measuring the amplitude from a trough to a crest.

Instead: The amplitude is measured from the rest position to a crest (or to a trough). Trough to crest is twice the amplitude.

Amplitude is the maximum displacement, and displacement is measured from where the material would be with no wave on it.

Mistake: Dividing a distance across several wavefronts by the number of wavefronts.

Instead: Divide by the number of gaps between them, which is one fewer. The first to the sixth crest is five wavelengths.

A wavelength is the space between neighbouring wavefronts, so you are counting spaces, not lines.

Mistake: Saying the frequency changes when water waves pass into shallower water.

Instead: The frequency stays the same. The speed and the wavelength decrease together.

The frequency is set by the source. Each wave that arrives at the boundary leaves it on the other side, so the number per second cannot change.

Mistake: Drawing diffracted wavefronts closer together, or further apart, than the incoming ones.

Instead: Keep the spacing the same. Only the shape of the wavefronts changes: they become curved.

The wave stays in the same medium, so its speed and frequency do not change, and so neither does its wavelength.

Mistake: Putting a wavelength in centimetres into $v = f λ$ and writing the answer in m/s.

Instead: Convert the wavelength to metres first, then the answer is in m/s.

Hz multiplied by cm gives cm/s. Labelling that number m/s makes the speed $100$ times too big.

Check yourself

A cork floats on a pond as ripples pass it. Describe the motion of the cork, and state what it shows about waves.
Answer
The cork bobs up and down as each ripple passes but is not carried along with the ripples. This shows that waves transfer energy without transferring matter.
A wave has a vertical distance of $8.0$ cm from a trough to a crest. State its amplitude, and explain why it is not $8.0$ cm.
Answer
$4.0$ cm. The amplitude is measured from the rest position to a crest, which is half of the trough-to-crest distance.
State what a wavefront is, and how the spacing of wavefronts is related to the wavelength.
Answer
A line joining points on a wave that are at the same stage of vibration, such as along a crest. The distance between neighbouring wavefronts is one wavelength.
Radio waves travel through air at $3.0 \times 10^{8}$ m/s. Calculate the wavelength of a radio wave of frequency $1.5 \times 10^{6}$ Hz.
Answer
$λ = v / f = (3.0 \times 10^{8}) \div (1.5 \times 10^{6}) = 200$ m
Sort these into waves that can be modelled as transverse and waves that can be modelled as longitudinal: sound, light, seismic P-waves, water waves, seismic S-waves.
Answer
Transverse: light, water waves, seismic S-waves. Longitudinal: sound, seismic P-waves.
In a ripple tank, straight waves pass at an angle onto a region where the water is shallower. State what happens to their speed, wavelength, frequency and direction.
Answer
Speed decreases, wavelength decreases, frequency stays the same, and the direction changes (towards the normal).
Water waves arrive at a gap $5.0$ cm wide. Which waves diffract more through it: waves of wavelength $0.50$ cm or waves of wavelength $4.0$ cm? Explain.
Answer
The $4.0$ cm waves, because their wavelength is closer in size to the width of the gap.
Describe how the wavelength of water waves affects how far they bend round the edge of a barrier.
Answer
The longer the wavelength, the more the waves diffract, so they bend further round the edge into the region behind the barrier.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student ties a short piece of ribbon to the middle of a long rope. She moves one end of the rope up and down repeatedly, and a wave travels along the rope.

(a) Explain why the wave on the rope is a transverse wave. 2 marks No calculator

Mark scheme

B1 the rope (or ribbon) vibrates up and down while the wave travels along the rope
B1 so the direction of vibration is at right angles to the direction of propagation, owtte

(b) Describe the motion of the ribbon as the wave passes it. 1 mark No calculator

Mark scheme

B1 moves up and down about a fixed position and does not move along the rope, owtte

(c) State what your answer to (b) shows about what a wave transfers. 1 mark No calculator

Mark scheme

B1 waves transfer energy without transferring matter (the rope is not carried along)

(d) The student replaces the rope with a long spring. Describe how she could send a longitudinal wave along the spring. 2 marks No calculator

Mark scheme

B1 push and pull the end of the spring back and forth along its length
B1 so the coils vibrate parallel to the direction the wave travels, forming compressions and rarefactions, owtte

Question 2. A vibrating bar in a ripple tank vibrates at $7.5$ Hz and makes straight water waves. The pattern is frozen, and the distance from the first crest to the ninth crest is $16$ cm.

(a) Calculate the speed of the waves. 3 marks

Mark scheme

C1 wavelength = 16 ÷ 8 = 2.0 cm or 0.020 m
C1 v = fλ, or 7.5 × 0.020 (or 7.5 × 2.0) seen
A1 0.15 m/s; unit required (15 cm/s also accepted)

(b) A glass plate is placed on the bottom of part of the tank, so the water above it is shallower. The waves travel more slowly over the plate. State what happens to the frequency and to the wavelength of the waves as they pass over the plate. 2 marks No calculator

Mark scheme

B1 frequency stays the same
B1 wavelength decreases

(c) Over the plate the speed of the waves is $0.090$ m/s. Calculate the wavelength of the waves over the plate. 2 marks

Mark scheme

C1 λ = v ÷ f, or 0.090 ÷ 7.5 seen, or a correct proportion method such as 2.0 × (0.090 ÷ 0.15)
A1 0.012 m (1.2 cm); unit required

Question 3. Straight water waves of wavelength $1.0$ cm in a ripple tank arrive at a barrier with a gap in it. The width of the gap can be changed.

(a) The gap is $1.0$ cm wide. Sketch three wavefronts beyond the gap. 2 marks No calculator

Mark scheme

B1 curved (near-semicircular) wavefronts spreading out from the gap
B1 spacing of the wavefronts the same as the wavelength of the incoming waves

(b) The gap is widened to $8.0$ cm. Predict how the waves beyond the gap will differ from those in (a). 2 marks No calculator

Mark scheme

B1 less diffraction / less spreading
B1 wavefronts mostly straight, curved only at their ends, owtte

(c) The gap stays at $8.0$ cm. Explain how the student could change the frequency of the vibrating bar to make the waves diffract more through the gap. 2 marks No calculator

Mark scheme

B1 decrease the frequency
B1 this increases the wavelength, so it is closer in size to the gap

(d) The barrier is replaced by a single straight barrier, and the waves pass its end. Describe how the wavelength of the waves affects their diffraction at the edge. 2 marks No calculator

Mark scheme

B1 the waves bend (curve) round the edge into the region behind the barrier
B1 longer wavelength gives more diffraction / waves bend further round, ORA

How this is examined

Working out wave speed from a ripple-tank measurement with $v = f λ$ : a two- or three-mark Calculate part, with the wavelength given in cm or as a distance across several wavefronts The equation $v = f λ$ recalled (Paper 4 does not give it); the wavelength found by dividing the distance by the number of gaps, one fewer than the number of wavefronts; cm converted to m; the answer with the unit m/s. A correct wavelength or a correct substitution is working a C mark can credit when the final answer is wrong.
What happens to water waves when they cross into shallower water: a two- or three-mark State or Describe part about a ripple tank with a plate on its bottom, which may be followed by a Calculate part for the new wavelength Speed decreases; wavelength decreases (wavefronts closer together); frequency unchanged. If the wavefronts meet the boundary at an angle, the direction changes, towards the normal. For the calculation, the unchanged frequency used with the new speed in $λ = v / f$ .
How gap width and wavelength decide how much a wave spreads: a two- or three-mark Describe or Predict part about changing a gap or a frequency, or a Sketch of the wavefronts beyond a gap A comparison of the two lengths: more diffraction when the gap is closer in size to the wavelength; a gap much wider than the wavelength gives little spreading, with wavefronts curved only at their ends; at an edge, a longer wavelength gives more diffraction. In a sketch: curved wavefronts beyond the gap with the same spacing as the incoming ones.
Telling transverse from longitudinal by comparing two directions: a one- or two-mark State or Describe part, which may ask you to Give an example Transverse: direction of vibration at right angles to the direction of propagation. Longitudinal: direction of vibration parallel to the direction of propagation. Both directions named. Examples: electromagnetic radiation, water waves and S-waves are transverse; sound and P-waves are longitudinal.
Showing that a wave moves energy but not the material it travels through: a one- or two-mark Describe or Explain part about a ribbon on a rope, a cork on water or a coil of a spring The object vibrates about a fixed position (up and down, or back and forth) and is not carried along with the wave; so the wave transfers energy without transferring matter.
Defining amplitude, wavelength and frequency exactly: one-mark State parts, or labels to add to a drawing of a wave Amplitude: maximum displacement from the rest position, not trough to crest. Wavelength: distance between neighbouring crests (the same point on neighbouring waves). Frequency: number of waves passing a point per second, in Hz.

Words to know

Wave: A travelling disturbance that transfers energy from place to place without transferring matter.
Rest position: Where the material would be if there were no wave passing through it.
Crest (peak): A highest point of a wave above the rest position.
Trough: A lowest point of a wave below the rest position.
Amplitude: The maximum displacement of a point on a wave from its rest position.
Wavelength (λ): The distance from a point on one wave to the same point on the next wave, such as crest to neighbouring crest; measured in m.
Frequency (f): The number of complete waves passing a point each second; measured in hertz (Hz).
Wave speed (v): The distance a wave travels each second; measured in m/s. $v = f λ$ .
Wavefront: A line joining points on a wave that are at the same stage of vibration, such as a line along a crest.
Transverse wave: A wave in which the direction of vibration is at right angles to the direction of propagation.
Longitudinal wave: A wave in which the direction of vibration is parallel to the direction of propagation.
Compression and rarefaction: In a longitudinal wave, a region where the particles or coils are bunched together, and a region where they are spread apart.
Reflection: The bouncing back of a wave from a surface, with the angle of incidence equal to the angle of reflection.
Refraction: A change in the direction of a wave caused by a change in its speed as it passes into a different medium.
Diffraction: The spreading out of a wave as it passes through a gap or past the edge of an obstacle.
Ripple tank: A shallow, transparent-bottomed tray of water in which a vibrating bar makes water waves that can be seen projected onto a screen.

All Physics lessons Next: 3.2.1 Reflection of light

3.1 General properties of waves

What a wave actually carries

The words that describe a wave

Wave speed: v=fλ

Transverse and longitudinal waves

Reflection, refraction and diffraction

The ripple tank: seeing all three

How much does a wave diffract?

Worked examples

Where people go wrong

Check yourself

Exam questions

How this is examined

Words to know

Wave speed: $v = f λ$