Learning outcomes
- Explain that waves transfer energy without a net transfer of matter.
- Describe wave motion using ropes, springs and water waves.
- Distinguish a particle’s vibration from the direction in which the wave travels.
- Use the ideas of wavefront and ray when describing two-dimensional waves.
1.1 What a wave is
A wave is a travelling disturbance that transfers energy from one place to another. The material through which a mechanical wave travels may move locally, but the material does not travel with the wave. For example, a floating cork on water moves mainly up and down while the wave pattern travels across the surface. After the wave has passed, the cork remains close to its original position. This observation is the key evidence that energy has moved without a net transfer of water.
Mechanical waves require particles that can interact. A disturbance makes one particle move, that particle exerts forces on neighbouring particles, and the disturbance is passed through the medium. Electromagnetic waves are different because they can travel through a vacuum. In both cases, the word “wave” describes a pattern that carries energy and information rather than a stream of material.
1.2 Wave motion in a rope and spring
If one end of a stretched rope is moved once up and down, a pulse travels along it. Each small part of the rope moves transversely for a short time and then returns towards its equilibrium position. Repeated regular motion of the hand creates a train of waves. Increasing the rate of vibration increases the frequency, while increasing the displacement of the hand increases the amplitude.
A long spring can demonstrate both main types of mechanical wave. Moving the end sideways produces a transverse disturbance. Pushing and pulling the end along the length of the spring produces moving compressions and rarefactions, which model a longitudinal wave. The coils themselves oscillate around mean positions; they do not travel from one end of the laboratory to the other.
1.3 Water waves and surface particles
Water waves are often described as transverse at this level because the visible displacement of the surface is perpendicular to the horizontal direction of travel. In reality, particles near the surface move in small circular or elliptical paths. The simplified transverse model remains useful for identifying crests, troughs, wavelength and amplitude.
When a water wave reaches an object, it can transfer energy to it. A floating object may bob or rock, and a large sea wave can do work on a shoreline. However, a labelled patch of water does not move forward at the wave speed. Any slow drift caused by currents must be distinguished from the oscillatory motion associated with the wave itself.
1.4 Wavefronts and rays
A wavefront is an imaginary line joining points on a wave that are in the same phase. A straight vibrating bar in a ripple tank creates approximately straight, parallel wavefronts. A point source creates circular wavefronts that spread outward. The spacing between neighbouring wavefronts is one wavelength.
A ray is an imaginary line showing the direction of energy transfer. Rays are drawn at right angles to wavefronts. They are especially useful when analysing reflection and refraction because they reduce a broad wave pattern to a clear direction. Wavefront diagrams are better when the question focuses on wavelength, speed changes or diffraction.
1.5 Energy, amplitude and damping
Greater amplitude generally means that the wave carries more energy. A large water wave can move an object more strongly than a small ripple, and a louder sound has a larger pressure amplitude than a quiet sound. At O Level, no general equation between wave energy and amplitude is required, but students should know the qualitative link.
Real waves lose useful energy as they travel because of friction, viscosity, electrical resistance or spreading. Their amplitude therefore decreases; this is called damping or attenuation. Damping does not mean that the wave suddenly stops. Instead, energy is transferred to the surroundings, usually as internal energy.
Worked examples
Following a pulse
A marker on a rope moves 4 cm upward and returns as a pulse passes. The marker does not move along the rope. Therefore the 4 cm movement is particle displacement, while the pulse travelling along the rope shows the direction of energy transfer.
Practical focus
Investigation
Stretch a rope across the room and attach small paper markers at intervals. Send one pulse along the rope and observe each marker. Record whether each marker moves along the rope or oscillates about one position. Repeat using larger and smaller pulses, keeping the tension similar.
Examination guidance
- Use the phrase “no net transfer of matter”; saying “particles do not move” is incorrect because they vibrate.
- Do not confuse the path of a particle with the path of the wave.
- When asked for evidence, describe what is observed, such as a cork returning to nearly the same place.
Check your understanding
- A leaf moves up and down as ripples cross a pond. What is transferred across the pond?
- Why is sound described as a mechanical wave?
- What is the relationship between a ray and a wavefront?
Answers
- Energy is transferred; the leaf and water have no net motion with the wave.
- Sound needs particles whose interactions pass the disturbance from one region to the next.
- A ray is perpendicular to the wavefront and shows the direction of energy transfer.
