SSC CGL 2025 Tier-I · 15 September 2025, Shift 2 · General Awareness · Question 16 of 100

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 11 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

What change occurs when sound passes from air into water?

(a)Frequency remains unchanged, but wavelength varies
(b)Frequency increases while wavelength stays constant
(c)Both frequency and wavelength decrease
(d)Speed decreases while frequency increases

Answer

Why

Correct — A. The frequency of a wave is set by its source, so it does not change when sound crosses from air into water.

The speed does change. NCERT's Class 9 table gives 1498 m/s in distilled water and 346 m/s in air at 25 °C, so sound is over four times as fast in water.

Wavelength = speed ÷ frequency.
Speed rises and frequency is fixed, so the wavelength grows by the same factor → option (a).

Why the others are wrong

(b)
Frequency increases while wavelength stays constant — Frequency cannot increase at the boundary, because the vibrating source fixes it. And with speed rising in water, v = f × λ forces the wavelength to change, not stay constant.
(c)
Both frequency and wavelength decrease — Frequency stays fixed, so it cannot decrease. The wavelength does change, but it increases, because sound speeds up in water and λ = v ÷ f.
(d)
Speed decreases while frequency increases — Both halves are wrong. Sound travels faster in water than in air, not slower, and its frequency stays the same across the boundary instead of increasing.

Concept

Three quantities describe a sound wave, linked by v = f × λ: speed equals frequency times wavelength.

The source fixes the frequency, the number of vibrations each second. Crossing into a new medium does not change that count.

The medium fixes the speed. In NCERT's Class 9 words, the speed of sound decreases when we go from solid to gaseous state.

So when the speed changes and the frequency cannot, the wavelength takes up the change.

The same rule holds for light entering glass or water: the frequency stays the same while speed and wavelength fall together.

Key facts

v = f × λ: speed equals frequency times wavelength.
Frequency depends on the source, and speed depends on the medium.
NCERT gives the speed of sound in air as 331 m/s at 0 °C and 344 m/s at 22 °C.
NCERT's table at 25 °C lists sound at 1498 m/s in distilled water and 346 m/s in air, over four times as fast in water.

Study next

Speed of sound in solids, liquids and gases, and the effect of temperature and humidityRefraction of light: why frequency stays fixed as light enters glassEcho and SONAR: distance = speed × time ÷ 2

Common traps

Assuming sound slows down in denser water, when it travels over four times as fast there.
Letting the frequency change at the boundary: it belongs to the source, not the medium.

v = f × λ drives a calculation at 11 Sep 2024, 16:00, GA Q.16: 5 kHz × 6 cm gives 300 m/s, so the sound covers 1.5 km in 5 seconds.

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 178 SSC CGL questions on it, every solved shift.