Lesson overview
Waves in a ripple tank: How do waves carry energy without carrying the water?
Predict what a ribbon on a slinky does as a wave passes, then send an up-and-down wave and a push-and-pull wave along it. In a ripple tank, watch a cork as waves cross the water. Freeze the picture to measure the wavelength at two frequencies, and work out the wave speed. Then lay in a barrier, a glass plate, a gap and an open end, and see the waves bounce off (reflect), change direction (refract) and spread out (diffract).
Suggested sequence
- Lesson 1 · Waves and how they move. Waves carry energy, not matter; transverse and longitudinal waves on a slinky; wavefront, crest, trough, amplitude, wavelength and frequency; the wavelength and wave speed measured in a ripple tank, with wave speed = frequency × wavelength. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Reflection, refraction and diffraction. Reflection, refraction and diffraction in a ripple tank: a barrier, shallow water, a gap and an edge; Supplement: how the gap size and the wavelength change diffraction at a gap and at an edge. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Waves and how they move
- 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 (3.1.1, 3.1.2)
- Know that for a transverse wave the direction of vibration is at right angles to the direction of propagation, and for a longitudinal wave it is parallel to the direction of propagation; understand that electromagnetic radiation, water waves and seismic S-waves (secondary) can be modelled as transverse, and sound waves and seismic P-waves (primary) as longitudinal (3.1.5, 3.1.6)
- 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 = fλ (3.1.3, 3.1.4)
Part 2: Reflection, refraction and diffraction
- Describe how waves can undergo reflection at a plane surface, refraction due to a change of speed and diffraction through a narrow gap (3.1.7)
- Describe the use of a ripple tank to show reflection at a plane surface, refraction due to a change in speed caused by a change in depth, diffraction due to a gap and diffraction due to an edge (3.1.8)
- Supplement: Describe how wavelength and gap size affects diffraction through a gap; describe how wavelength affects diffraction at an edge (3.1.9, 3.1.10)
Misconceptions to listen for
- “It is carried along with the ripples to the edge of the pond.” — A wave carries energy, not the water. The water, and the leaf with it, rises and falls and ends up where it started.
- “It stays completely still while the ripples pass under it.” — The energy passing through makes the water rise and fall where it is, so the leaf moves too.
- “It is pushed back towards the place where the stone landed.” — Nothing pushes the water back. Each bit of water only rises and falls about its own place as the ripples pass.
- “Row B.” — The columns are swapped. Tipping the cup needs something to reach the far end: the energy. The ladybird back in its place shows that the hose under it was not carried along.
- “Row C.” — The ladybird moving sideways happens halfway along, not at the far end, and the cup tipping over shows that something arrived, not that the hose stayed in place.
- “Row D.” — The cup tipping over does show that energy arrived. But moving sideways on its own does not show the hose stayed in place: ending where it started does.
- “The frequency is 8.0 s ÷ 12 = 0.67 Hz.” — That is the time for one vibration. The frequency is the number of vibrations each second.
- “The wave speed is the wavelength divided by the frequency.” — The wave speed is v = fλ: the frequency times the wavelength.
- “Seismic P-waves are transverse and S-waves are longitudinal.” — It is the other way round: P-waves (primary, they arrive first) are longitudinal, like sound; S-waves (secondary) are transverse.
- “Sound is transverse, like a wave on a rope.” — In sound the air particles vibrate to and fro along the direction the sound travels, so sound is longitudinal.
Scope
Waves transfer energy without transferring matter (3.1.1); wave motion on a spring and in a ripple tank (3.1.2); wavefront, wavelength, frequency, crest, trough, amplitude and wave speed (3.1.3); wave speed = frequency × wavelength (3.1.4); transverse and longitudinal waves, with examples (3.1.5, 3.1.6); reflection at a plane surface, refraction by a change of speed and diffraction through a narrow gap (3.1.7), shown in a ripple tank, with refraction by a change of depth and diffraction at a gap and an edge (3.1.8); Supplement: how the gap size and the wavelength affect diffraction at a gap (3.1.9) and at an edge (3.1.10). The law of reflection with angles is in Reflection in mirrors, Snell’s law in Refraction and total internal reflection, and the electromagnetic spectrum and sound have their own modules.