Tides, Waves & Tsunamis – NDA Geography PYQs

Question 1 [NDA 2007-II]
Consider the following statements:
Assertion: The tidal effect of Moon on Earth is less than that of Sun.
Reason: Moon’s gravitational pull at Earth’s surface is less compared to Sun’s.

(a) Both A and R true and R explains A
(b) Both true but R not explanation of A
(c) A true but R false
(d) A false but R true

Answer: (d) A false but R true
The Assertion is FALSE — the Moon’s tidal effect on Earth is GREATER than the Sun’s (about 2.2 times stronger) despite the Sun being far more massive. This is because tidal force depends on the gradient of gravity across the Earth’s diameter — and the Moon, being much closer, creates a steeper gradient. The Reason is TRUE — the Moon’s total gravitational pull at Earth’s surface is indeed less than the Sun’s — but this does not make the assertion true. So A is false; R is true.

---

Question 2 [NDA 2010-I]
Which one of the following best describes why movements of tides are mostly determined by?
(a) Wind
(b) Gravitational force of Moon
(c) Rotation of the Earth
(d) Gravitational force of Sun

Answer: (c) Rotation of the Earth
The Moon and Sun create tidal bulges in the ocean. But the timing and pattern of tides at any given location — why high tide comes when it does — is determined by the Earth’s rotation carrying the surface through these bulges and troughs. The Moon creates the bulge; Earth’s rotation moves the ocean into and out of the bulge.

Question 3 [NDA 2010-I]
Interval between two high tides is approximately:
(a) 4 hours
(b) 6 hours
(c) 12 hours
(d) 24 hours

Answer: (c) 12 hours
Most coastal locations experience two high tides per day. The interval between two successive high tides is approximately 12 hours (more precisely 12 hours 26 minutes). NDA accepted 12 hours as the approximate answer in this question.

---

Question 4 [NDA 2011-I]
The exceptionally high and low tides at the time of New Moon or Full Moon when Sun, Moon and Earth are approximately aligned are called:
(a) Spring tides
(b) Neap tides
(c) Equinoctial tides
(d) Diurnal tides

Answer: (a) Spring tides
Spring tides occur when the gravitational pulls of the Sun and Moon combine — at New Moon (conjunction) and Full Moon (opposition) when the three bodies are aligned. They produce the highest high tides and lowest low tides — the maximum tidal range.

---

Question 5 [NDA 2013-I]
As a tsunami wave leaves deep water and travels into shallow water:

1. Its speed considerably increases.
2. They attain enormous height.
3. They appear as a gentle rise and fall of sea.

Which of the above is/are correct?
(a) 2 only
(b) 1 and 3
(c) 2 and 3
(d) 1 and 2

Answer: (a) 2 only
Statement 1 is wrong — speed DECREASES (not increases) as tsunami enters shallow water. Statement 2 is correct — height increases dramatically as the wave energy is concentrated in shallower, shorter wavelength water. Statement 3 is wrong — in shallow water they appear as enormous destructive walls of water, not gentle rises and falls (they appear as gentle rises and falls only in the DEEP ocean).

Question 6 [NDA 2019-I]
Spring tides refer to:
(a) Greatest difference in sea level at high and low tides
(b) Lowest difference in sea level at high and low tides
(c) No difference in sea level at high and low tides
(d) Counteraction of gravitational pulls

Answer: (a) Greatest difference in sea level at high and low tides
Spring tides have the maximum tidal range — the biggest difference between high and low water levels. Neap tides have the minimum tidal range. Spring tides are the strongest tides; neap tides are the weakest.

---

Question 7 [NDA 2019-I]
The time gap between two successive tides at a given place is approximately:
(a) 12 hours
(b) 12 hours 26 minutes
(c) 24 hours
(d) 24 hours 52 minutes

Answer: (b) 12 hours 26 minutes
The Moon advances in its orbit by about 12° per day. The Earth must rotate this extra distance to bring the same point back under the Moon — adding about 50 minutes per full tidal cycle, or about 26 minutes per high tide. This precise answer distinguishes from the approximate 12 hours given in NDA 2010-I.

---

Question 8 [NDA 2023-I]
Which positions of Sun, Earth and Moon are suitable for Spring Tide?

1. Syzygy Conjunction
2. Syzygy Opposition
3. Quadrature

(a) 1 only
(b) 2 only
(c) 1 and 2
(d) 1, 2 and 3

Answer: (c) 1 and 2
Syzygy Conjunction (New Moon — Moon between Sun and Earth) and Syzygy Opposition (Full Moon — Earth between Sun and Moon) both create spring tides — because in both cases the gravitational pulls of Sun and Moon are aligned. Quadrature (Moon at 90° to Sun-Earth line) creates NEAP tides — the weakest tides.

Question 9 [NDA 2024-I]
Consider the following statements about Neap Tides:

1. Neap tides occur every 14–15 days.
2. The tidal range is small because gravitational forces of Moon and Sun are in quadrature.

Which is/are correct?
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (c) Both 1 and 2
Both statements are correct. Neap tides occur approximately every 14–15 days — at the First Quarter and Last Quarter of the lunar cycle. The tidal range during neap tides is small because the Moon’s gravitational pull and the Sun’s gravitational pull are at right angles (quadrature) — partially cancelling each other rather than combining.

---

Question 10 [NDA 2025-II]
Consider the following statements about sea waves:

1. Height of waves is determined by wind speed, wind duration, and fetch.
2. Energy of a wave is proportional to the square of its height.
3. Waves retain most of their energy as they travel across the deep ocean.

Which is/are correct?
(a) 1 only
(b) 1 and 2
(c) 2 and 3
(d) 1, 2 and 3

Answer: (d) 1, 2 and 3
All three statements correctly describe ocean wave properties. Wave height is controlled by wind speed, duration, and fetch. Wave energy is proportional to the square of wave height — so doubling wave height quadruples its energy. Waves in the deep ocean lose very little energy as they travel — they can cross entire ocean basins.