CAPF AC 2020 Q65: Geography – Oceanography
The distance between two successive crests or troughs of sea waves is called as
- (a) Wavelength
- (b) Wave height
- (c) Wave frequency
- (d) Fetch
Answer: (a) Wavelength
Explanation
Parts of a sea wave
How to think: the question asks for a distance along the water, from one crest to the next.
A horizontal length of a wave is its wavelength.
| Term | Meaning | Direction |
|---|---|---|
| Wavelength | Distance between two successive crests or two successive troughs | Horizontal |
| Wave height | Distance from the bottom of a trough to the top of a crest | Vertical |
| Wave frequency | Number of waves passing a point in one second | A count, not a distance |
| Fetch | Distance of open water over which the wind blows | Horizontal, but about the wind |
More terms:
| Term | Meaning |
|---|---|
| Crest | Highest point of a wave |
| Trough | Lowest point of a wave |
| Wave period | Time between two successive crests passing a point |
| Amplitude | Half of the wave height |
Answer: (a) Wavelength.
Why each option is right or wrong
- (a) CORRECT. Wavelength is the horizontal distance between two successive crests or troughs.
- (b) Incorrect. Wave height is the vertical distance from trough to crest.
- (c) Incorrect. Wave frequency is the number of waves passing a point each second.
- (d) Incorrect. Fetch is the stretch of open water over which the wind blows.
Key point
Wavelength: crest to crest (horizontal). Wave height: trough to crest (vertical). Amplitude: half the height. Fetch: distance of open water the wind blows over.
Remember
Crest to crest = wavelength; trough to crest = wave height.
Common mistakes
- Mixing up wavelength (horizontal) with wave height (vertical).
- Thinking fetch is a part of the wave; it is about the wind's run over water.
- Confusing wave period (time) with frequency (number per second).
Exam tip
Crest to crest is a length, so wavelength. Crest to trough is a height.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography (Movements of Ocean Water)
- NCERT Class IX Science — Sound (wavelength, frequency, amplitude)