The distance between two successive crests or troughs of sea waves is called as

Correct — A, Wavelength. Wavelength is the horizontal distance between two successive points in the same phase of a wave, which for a sea wave means crest to crest or trough to trough. The other three terms in the options are all real oceanographic quantities, and each measures something different — one a vertical distance, one a rate, and one a length of open water rather than a property of the wave at all.

• (b)Wave height — A vertical measurement, not a horizontal one. Wave height is the difference in level between the trough and the crest, and the amplitude is half of it.
• (c)Wave frequency — A rate rather than a distance — the number of waves passing a fixed point in unit time. Its reciprocal is the wave period, the time between two successive crests.
• (d)Fetch — The length of open water over which the wind blows without interruption. Fetch helps determine how large the waves become, but it is a property of the sea area and not a dimension of the wave itself.

A wave is described by four quantities that lock together. Wavelength is the distance from one crest to the next, period is the time between them, frequency is the reciprocal of the period, and speed is wavelength divided by period, which is the same as wavelength multiplied by frequency. Height, measured trough to crest, is independent of those four and carries the energy — wave energy rises roughly with the square of the height.

Wind waves in the open sea are built by three things: how hard the wind blows, how long it blows for, and the fetch. That is why option (d) is placed here — a candidate who has learnt fetch as part of the wave chapter may reach for it without noticing that it describes the sea and not the wave. The contrast with a tsunami is worth carrying. A tsunami has an enormous wavelength, often more than a hundred kilometres, and a height in the deep ocean of barely a metre, which is why ships at sea do not notice one passing; the height builds only as the wave shoals near the coast and the same energy is packed into shallower water. Distinguishing wavelength from height explains that behaviour in a sentence.

• Wavelength is the horizontal distance between successive crests or successive troughs.
• Wave height is the vertical distance from trough to crest; amplitude is half the height.
• Wave period is the time between two successive crests, and frequency is its reciprocal.
• Wave speed equals wavelength divided by period, or equivalently wavelength multiplied by frequency.
• Fetch is the uninterrupted stretch of open water over which the wind blows, and along with wind speed and duration it sets how large waves grow.

• Mixing a horizontal measurement with a vertical one; wavelength and height answer different questions.
• Reading frequency as a distance because it appears in the same formula.
• Treating fetch as a property of a wave rather than of the water surface the wind crosses.

Definition items in this paper offer three genuine terms from the same chapter. Sort them first by what kind of quantity each is — a length, a time, a rate — and the answer usually appears without further thought.

• practice — not a real PYQ

  The time interval between the passage of two successive crests past a fixed point is called the

  • (a)wavelength
  • (b)wave period
  • (c)wave height
  • (d)fetch

  Answer(b) wave period — its reciprocal is the wave frequency.

• practice — not a real PYQ

  A wave of wavelength 20 metres has a frequency of 0·5 hertz. Its speed is

  • (a)5 metres per second
  • (b)10 metres per second
  • (c)20 metres per second
  • (d)40 metres per second

  Answer(b) 10 metres per second — speed equals wavelength multiplied by frequency, so 20 × 0·5 = 10.