GeographyCBSE Class 11 · Movements of Ocean Water

Waves and Tides of the Ocean (Class 11 Geography, Fundamentals of Physical Geography, Chapter 13)

Updated 17 October 2026 · Fact-checked

Waves are energy moving across the surface, with water particles moving in small circles; they break when depth is less than half the wavelength. Tides are the periodic rise and fall of sea level caused mainly by the moon and sun: spring tides occur at full and new moon when the three bodies are in line, neap tides about seven days later when they are at right angles.

Understand Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance

Why this topic matters. Wave terms, the causes of tides and spring-neap comparisons are asked as definitions, diagrams and short numericals.

1. Movements of ocean water. Horizontal: currents and waves. Vertical: tides, upwelling and sinking. In a wave the water does not move ahead; the wave train does.

2. Waves. Waves are energy, not water, moving across the surface; water particles move in small circles. Wind gives the energy. A breeze of 2 knots or less over calm water makes ripples, growing until white caps appear. Waves slow near the beach through friction with the sea floor and break when depth is less than half the wavelength. Steep waves are young and probably from local wind; slow, steady waves come from far away. Maximum wave height depends on wind strength, duration and the area over which it blows in one direction.

3. Wave characteristics. Crest and trough; wave height (vertical distance from trough to crest); amplitude (half the wave height); wave period (time between two successive crests passing a point); wavelength (horizontal distance between two successive crests); wave speed (in knots); frequency (waves passing a point in one second).

4. Tides. The periodic rise and fall of sea level once or twice a day, mainly because of the moon and, less so, the sun. Surges are irregular movements caused by weather. The moon's gravitational pull and the centrifugal force together create two bulges: on the side facing the moon the pull exceeds centrifugal force, on the opposite side centrifugal force dominates. Bulges are higher on wide continental shelves and in funnel-shaped bays and estuaries. The Bay of Fundy (Nova Scotia, Canada) has the highest tides in the world (bulge 15 to 16 m).

5. Types by frequency. Semi-diurnal (two highs and two lows a day, the most common), diurnal (one high and one low) and mixed (varying heights, for example the west coast of North America).

6. Types by position. Spring tides: sun, moon and earth in a line, higher tides, at full moon and new moon (twice a month). Neap tides: about seven days later, sun and moon at right angles, smaller range. At perigee the moon is closest (greater range); at apogee it is farthest (smaller range). At perihelion (around 3 January) ranges are greater; at aphelion (around 4 July) smaller.

7. Importance. Tides are predictable, help navigation (especially in estuaries with shallow bars), remove sediments and polluted water from estuaries, and give tidal power (Canada, France, Russia, China; a 3 MW project at Durgaduani in the Sunderbans).

Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance: key facts

Wave terms: $H e i g h t = t r o ug h t ocr es t ∣ A m pl i t u d e = ha l f t h e h e i g h t ∣ P er i o d = t im e b e tw ee n cr es t s ∣ W a v e l e n g t h = cr es tt ocr es t ∣ F r e q u e n cy = w a v es p er seco n d$; Wave breaks when depth is under half the wavelength.
Tide types: Semi-diurnal 2 highs, 2 lows
Diurnal 1 high, 1 low
Mixed varying heights; By frequency.
Spring versus neap: Spring: sun, moon, earth in line, full and new moon, higher | Neap: right angles, about 7 days later, smaller range; Twice a month each.
Distance effects: Perigee greater range
Apogee smaller range
Perihelion about 3 January greater
Aphelion about 4 July smaller; Moon and sun distance.

Semi-diurnal	2 highs, 2 lows
Diurnal	1 high, 1 low
Mixed	varying heights

Perigee	greater range
Apogee	smaller range
Perihelion about 3 January	greater
Aphelion about 4 July	smaller

How to answer Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance questions

Wave numericals use the definitions directly.

1Write height, period or wavelength from the data.
2Amplitude = height / 2; frequency = 1 / period.
3Check the breaking condition: depth less than wavelength / 2.
4State the answer with units.

Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Waves at the pier

Suppose waves of 2.4 m height pass a pier every 10 seconds.

Find the amplitude and the frequency.

Try it first, then show the solutionSolution

Amplitude is half the height: 2.4 / 2 = 1.2 m.
Frequency = 1 / 10 = 0.1 waves per second.

Answer: Amplitude 1.2 m; frequency 0.1 per second.

Memory hook: Half the height; one over period.

Fishing boat plan

Suppose a fisherman knows that tomorrow is a new moon and wants to plan a trip through a shallow estuary mouth.

What tide should he expect and why does it matter?

Try it first, then show the solutionSolution

New moon means sun, moon and earth in a line: a spring tide.
A higher tide helps ships cross shallow bars.

Answer: A spring tide, with a higher water level at the estuary.

Memory hook: New or full moon, spring.

Tricks to remember Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance

Short memory aids. Say each one aloud once, then use it on a question the same day.

Spring is straight
Sun-moon-earth in a straight line gives spring tides
Use it when: Spring.
Neap is right angle
Sun and moon at right angles reduce the tide
Use it when: Neap.
Half the wavelength
Waves break when depth is less than half the wavelength
Use it when: Breaking.

Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Amplitude · 1 mark · MCQBoard level

If a wave has a height of 4 m, its amplitude is:
(A) 1 m
(B) 2 m
(C) 4 m
(D) 8 m

Try it first, then show the solutionSolution

Amplitude is half the wave height: 2 m (1 mark).

Answer: (B)

Example 2: Spring tide · 2 marks · Very short answerBoard level

When do spring tides occur and what is their position? (2 marks)

Try it first, then show the solutionSolution

At full moon and new moon, twice a month (1 mark).
Sun, moon and earth lie in a straight line (1 mark).

Answer: Spring tides occur twice a month at full moon and new moon, when the sun, moon and earth are in a straight line, producing higher tides.

Example 3: Two tidal bulges · 3 marks · Short answerHarder board level

Explain why two tidal bulges form on opposite sides of the earth. (3 marks)

Try it first, then show the solutionSolution

Moon-facing side: moon's pull exceeds centrifugal force, bulge towards the moon (1 mark).
Far side: pull is weaker, centrifugal force dominates, bulge away from moon (1 mark).
Tide-generating force is the difference between gravity and centrifugal force (1 mark).

Answer: On the side facing the moon the moon's gravitational pull is greater than the centrifugal force, so water bulges towards the moon. On the opposite side the pull is weaker because it is farther from the moon and centrifugal force dominates, so water bulges away from the moon. The tide-generating force is the difference between the moon's attraction and centrifugal force.

Example 4: Assertion and reason: spring tide · 1 mark · Assertion-reasonHarder board level

Assertion (A): The highest tides of a month occur at full moon and new moon.
Reason (R): At these times the sun, moon and earth are in a straight line and their pulls add up.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

In-line positions add the pulls of sun and moon (1 mark).

Answer: (A)

Example 5: HOTS: perigee and spring tide · 5 marks · Long answerHOTS / competency

A coastal town expects a new moon, with the moon at perigee, and a few days later perihelion. Predict the likely tidal range, explain each factor, and say what precaution a harbour authority should take. (5 marks)

Try it first, then show the solutionSolution

New moon means sun, moon and earth in line: spring tide, higher range (1 mark).
Perigee: moon closest, unusually high and low tides, greater range (1 mark).
Perihelion (about 3 January): earth closest to the sun, much greater ranges (1 mark).
Together the tidal range is expected to be very large (1 mark).
Harbours and estuaries should schedule ship movements around higher tides and prepare for unusually low and high water (1 mark).

Answer: A new moon puts the sun, moon and earth in a line, giving spring tides with a higher range. Perigee brings the moon closest, giving unusually high and low tides, and perihelion (around 3 January) adds a much greater range. All three factors act in the same direction, so a very large tidal range is expected. The harbour authority should plan ship movements around the high tides and be ready for unusually low water over shallow bars.

Where students lose marks in Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance

The slips examiners see most often, and what to write instead.

Mistake: Giving amplitude equal to wave height
Terms mixed.
Wrong working
Amplitude = wave height.
Right working
Amplitude is half the wave height.
Do this: Divide by two.
Usually costs: 1 mark.
Mistake: Saying spring tides occur in spring
Name misleading.
Wrong working
Spring tides come in the spring season.
Right working
Spring tides occur at full and new moon, twice a month.
Do this: Link with moon phase.
Usually costs: 1 mark.
Mistake: Ignoring centrifugal force
Only the moon mentioned.
Wrong working
Tides are caused by the moon only.
Right working
Moon and sun gravity together with centrifugal force.
Do this: Mention both forces.
Usually costs: 1 mark.

Exam tips

Draw the earth-moon-sun diagram for spring and neap tides.
Give wave terms with a labelled sketch.
Quote the Bay of Fundy 15 to 16 m figure.

Practice questions on Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The place with the highest tides in the world is the:
(A) Bay of Bengal
(B) Bay of Fundy
(C) Gulf of Mexico
(D) Arabian Sea
Hint
Nova Scotia.
Answer
(B)
Q2 · 2 marks · Very short answerBoard level
Define wavelength and wave period. (2 marks)
Hint
Distance and time.
Answer
Wavelength is the horizontal distance between two successive crests; wave period is the time between two successive crests (or troughs) passing a fixed point.
Q3 · 3 marks · Short answerHarder board level
Swell of wavelength 84 m reaches a gently sloping shore. Below what depth of water will the waves begin to break and what causes this? (3 marks)
Hint
Half the wavelength.
Answer
Waves break when the water depth is less than half the wavelength, i.e. less than 42 m. Near the beach friction with the sea floor slows them, and they break as surf.
Q4 · 3 marks · Short answerHarder board level
Distinguish semi-diurnal, diurnal and mixed tides. (3 marks)
Hint
Highs per day.
Answer
Semi-diurnal tides have two highs and two lows daily of similar height and are the most common. Diurnal tides have one high and one low daily. Mixed tides vary in height, as on the west coast of North America and many Pacific islands.
Q5 · 4 marks · Short answerHOTS / competency
In a funnel-shaped estuary the tide rises 9 m in 6 hours. Find the average rate of rise in cm per hour and explain why such estuaries have large tides and why tides help ships there. (4 marks)
Hint
Convert to cm.
Answer
9 m = 900 cm; 900 / 6 = 150 cm per hour. Funnel-shaped bays and estuaries magnify tides because the water is squeezed into a narrowing channel. Tidal heights help ships cross shallow bars at the entrance, which block them at low water.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the wave period?Show answer
Time between two successive crests passing a point.
2.What are surges?Show answer
Irregular weather-driven water movements.
3.When is apogee?Show answer
When the moon is farthest.
4.What is ebb?Show answer
Falling water from high to low tide.

Ocean Waves and Tides: Wave Characteristics, Causes and Types of Tides, and Their Importance: frequently asked questions

Why do waves break?

Because depth becomes less than half the wavelength.

What are tidal currents?

Tides channelled between islands or into bays and estuaries.

How often are spring tides?

Twice a month.