What are wavefronts?
4 marks · HardA group of students investigate waves on two ropes by moving the ends back and forth. One rope's end is vibrated up and down at right angles to the rope; the other is vibrated backwards and forwards along its length.
State which student produces a transverse wave and which produces a longitudinal wave. Explain your reasoning with reference to vibration direction.
For either wave, describe what a wavefront is and how you could identify wavefronts in the rope.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Transverse: the up-and-down vibration; Longitudinal: along the rope's length. Reason: Transverse waves have vibrations perpendicular to energy transfer; longitudinal waves have vibrations parallel. · must show Correct assignment of wave type, Reference to vibration and energy directions
(b) A wavefront is a line where all particles vibrate in phase and are the same distance from the wave source. On the rope, wavefronts can be identified by marking points oscillating together at the same stage in vibration. · must show Definition of wavefront, Method for identifying wavefronts
Common mistakes: Mixing up wave directions; Assuming wavefront is always a crest
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly identifies which movement produces transverse/longitudinal wave (matching context).
M2 (a) · 1 mark · Explains the direction of vibration relative to energy transfer for each wave type.
B1 (b) · 1 mark · Defines a wavefront correctly: points in phase/at same stage.
B2 (b) · 1 mark · Describes a valid way to identify or visualize wavefronts on a rope.
Worked solution
(a) The student who vibrates the rope up and down produces a transverse wave, because the vibrations are at right angles to the direction in which the energy travels. The student vibrating the rope along its length produces a longitudinal wave because the vibrations are parallel to the energy direction.
(b) A wavefront is a line joining points on a wave that are in phase — at exactly the same stage of the vibration and the same distance from the source. On a rope, you could use a strobe light or high-speed camera to look for parts of the rope that move up and down together (for transverse) or compress and stretch together (longitudinal), marking points that are at the crest or trough at the same instant.