Maarg Manthan · Topic 6.8

Waves: Wavelength, Amplitude, Frequency

Geography › Oceanography · Topic 6.8

Waves are the up-and-down movement of the sea surface, produced mainly by wind blowing over the water. A wave is described by its crest (top), trough (bottom), height (from trough to crest), wavelength (distance between two crests), period (time between two crests) and frequency (number of waves per second). Amplitude is half the wave height. Waves carry energy, not water: the water particles move in circles, and the wave form travels. When waves reach shallow water they slow, grow steeper and break, which shapes coasts.

5 min read · UPSC Prelims · Geography · Oceanography

At a Glance

Wavelength Distance between two crests
Amplitude Half the wave height
Period Time between two crests
Frequency Waves passing a point per second
Cause Mainly wind
Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This topic follows the one on ocean currents and describes the second movement of ocean water, the surface waves. Tides and tsunamis are long waves of a different origin, and are covered in other topics. Previous topic: Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents. Next topic: Tides: Tidal Currents, Types and Importance.

What Are the Parts of a Wave?

Term	Meaning
Crest	The highest point of the wave
Trough	The lowest point of the wave
Wave height	Vertical distance from trough to crest
Amplitude	Half the wave height, measured from the still-water level
Wavelength	Horizontal distance between two successive crests (or troughs)
Wave period	Time taken for two successive crests to pass a point
Frequency	Number of waves passing a point in a second; the inverse of the period
Wave speed	Wavelength divided by the period

In deep water, the water particles move in circles, with the radius falling quickly with depth, and the motion dies out at a depth of about half a wavelength, called the wave base. The wave moves forward, but the water does not move with it, as can be seen when a floating cork bobs up and down.

How Are Waves Made?

Wind waves: the friction of wind on the water creates ripples, and then larger waves as the wind goes on blowing.
Controls: the size of a wind wave depends on the wind speed, how long it blows (duration) and the distance of open water it blows across (fetch). Large oceans with strong winds, as in the Southern Ocean, make the largest waves. The largest wave measured by an instrument, in the North Atlantic in 2013, was about 19 metres high.
Swell: waves that leave the area where they were made and travel far, becoming long, regular and smooth. Swell from southern storms reaches the coasts of India in the monsoon.
Other waves: tsunamis, caused by sudden sea-floor movement, have very long wavelengths of up to hundreds of km, as in Tsunami: Mechanism, Properties and Effects. Storm surges are raised by cyclones. Seiches are waves that slosh in an enclosed basin. Internal waves occur below the surface along the pycnocline.

What Happens When Waves Reach the Coast?

When a wave enters water shallower than about half its wavelength, the bottom drags on it. It slows down, the wavelength shortens, and the height increases. When the water depth is about 1.3 times the wave height, the wave becomes unstable, the crest topples forward and the wave breaks. Breakers are of three kinds: spilling breakers on gentle beaches, plunging breakers on steeper beaches and surging breakers on very steep shores. The uprush of water on the beach is the swash, and the return flow is the backwash. Waves approaching at an angle are bent towards the shore (refraction), so they concentrate energy on headlands and spread it in bays. Their work in cutting and building coasts is in Coastal Erosional Landforms: Cliffs, Terraces, Caves, Stacks and Coastal Depositional Landforms: Beaches, Dunes, Bars, Barriers, Spits.

Why Do Waves Matter?

Coasts: waves are the main agent shaping beaches, cliffs, spits and bars.
Navigation and ports: high waves delay ships and damage ports, so breakwaters are built.
Mixing: waves stir the surface layer and mix oxygen into the water.
Energy: the power in waves is a possible renewable source. India’s wave energy potential along its coast is often estimated at tens of thousands of megawatts, but has been little used so far, and the figures should be checked in official reports.
Hazards: rogue waves, which are much higher than the waves around them, and cyclone waves endanger ships and fishing boats.

As an analytical point, waves are surface movements that transfer energy from wind to shore, unlike the currents that move water over long distances, and unlike the tides that rise and fall regularly.

Why Is This Topic Important?

Terms Wave terms are asked
Coast Basis of coastal processes
Energy Renewable potential

What Came Next?

The next topic describes tides, the third regular movement of the oceans. Next topic: Tides: Tidal Currents, Types and Importance.

Key People and Terms

Crest Top of a wave
Trough Bottom of a wave
Fetch Distance wind blows over water
Swell Long regular waves from distant storms
Wave base Depth where wave motion ends
Breaker Wave that topples near the shore

Exam Corner

Points to Remember

Amplitude is half the wave height.
Wave speed = wavelength divided by period.
Waves transfer energy, not water.
Wave size depends on wind speed, duration and fetch.
Waves break when depth is about 1.3 times the wave height.

Do Not Confuse With

Wave height and amplitude Trough to crest versus half of that
Wavelength and period Distance versus time
Waves and tides Wind-made surface waves versus gravity-made rise and fall

Memory Hook

Crest to crest is length, time is period, half the height is amplitude.

Mains Angle

Questions on waves are mostly factual at Prelims. These points add depth for a short answer.

Parts Wavelength, amplitude, frequency
Origin Wind, fetch, duration
Coast Breaking and refraction

A Question You May Face

An original practice question, not a past paper question.

“Describe the characteristics of ocean waves and explain how they change as they approach the shore.”

How to Answer

Parts Crest to wavelength.
Origin Wind and fetch.
Shore Slowing, breaking, refraction.
Use Energy and hazards.

GS Relevance

Prelims: parts of a wave, wavelength, period, frequency, swell, fetch, breakers, tsunami waves. GS Paper 1: oceanography.

Frequently Asked Questions

What are the main characteristics of an ocean wave?

An ocean wave has a crest at the top and a trough at the bottom. Its height is the vertical distance between them, its amplitude is half of that, its wavelength is the distance between two crests, its period is the time between crests, and its frequency is the number of waves passing a point per second.

What is the difference between wavelength and amplitude?

Wavelength is the horizontal distance between two successive crests or troughs of a wave, while amplitude is half the vertical distance between the trough and the crest. The wavelength shows how long the wave is, and the amplitude indicates how much energy it carries.

What is fetch?

Fetch is the distance of open water over which the wind blows in a constant direction and builds up waves. Along with the wind speed and its duration, the fetch decides the size of a wind wave, so long fetches in the open ocean produce the largest waves.

What is swell?

Swell is the regular, smooth, long-wavelength waves that have travelled out of the storm area where they were generated. Because of their long wavelength and low steepness, they can travel thousands of kilometres across an ocean with little loss of energy, and arrive on distant coasts.

Why do waves break near the shore?

As a wave enters shallow water, friction with the sea floor slows its lower part, so the wavelength shortens and the height grows. When the depth is about 1.3 times the wave height, the crest outruns the base and topples forward, so that the wave breaks as a breaker on the shore.

PYQ Practice — Statement Analysis

1 The amplitude of a wave is half of its height.

True

Height is measured from trough to crest.

2 Waves transport water over long distances with the wave form.

False

They transfer energy, not water.

3 Fetch is the distance over which wind blows across open water.