SSC CGL 2025 Tier-I · 13 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 13 September 2026.

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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.

When light enters from air into a medium, its speed changes. Which of the following properties of light remains constant during this transition?

(a)Wavelength
(b)Frequency
(c)Speed
(d)Amplitude

Answer

Why

Correct — B. Frequency is set by the source, not by the medium the light travels through.

At the boundary, every crest that arrives from the air must continue into the new medium, so the number of crests passing per second cannot change → option (b).

What changes follows from v = fλ: with f fixed, a lower speed means a proportionally shorter wavelength.

Why the others are wrong

(a)
Wavelength — Wavelength changes with speed. With f fixed in v = fλ, light entering glass (n ≈ 1.5) slows to about 2 × 10⁸ m/s and its wavelength shrinks by the same factor, 1.5.
(c)
Speed — Speed is the property the stem itself says changes. In a medium v = c⁄n, so in water (n ≈ 1.33) light travels at about 2.25 × 10⁸ m/s instead of 3 × 10⁸ m/s.
(d)
Amplitude — Amplitude is not protected at a boundary. Part of the light is reflected at the surface and part absorbed inside the medium, so the transmitted wave usually has a smaller amplitude.

Concept

Refraction is the bending of light as it passes from one medium into another, and it happens because light changes speed at the boundary.

The refractive index of a medium is n = c⁄v, light's speed in vacuum divided by its speed in the medium. Air is close to 1, water about 1.33, glass about 1.5.

Since frequency stays fixed, the wavelength inside a medium is the vacuum wavelength divided by n.

Colour is tied to frequency, so light does not change colour when it enters water or glass, even though its wavelength there is shorter. Sound obeys the same rule: its frequency stays fixed across a boundary while its speed and wavelength change.

Key facts

On refraction, frequency stays the same while speed and wavelength change.
v = fλ links speed, frequency and wavelength.
Refractive index n = c⁄v: about 1.33 for water and about 1.5 for glass.
The speed of light in vacuum is about 3 × 10⁸ m/s.

Study next

Snell's law: n₁ sin i = n₂ sin rTotal internal reflection and the critical angleDispersion: why violet bends more than red in a glass prism

Common traps

Choosing wavelength because a wave's 'size' seems fixed: wavelength shrinks in step with speed.
Assuming frequency falls because the light slows down: speed and wavelength fall together while frequency stays put.

The same rule for sound is asked at 15 Sep 2025, 12:30, GA Q.16 (keyed: frequency unchanged, wavelength varies from air into water), and v = fλ as a sum at 16 Sep 2025, 16:00, GA Q.18 (340 ÷ 680 = 0.5 m).

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 220 SSC CGL questions on it, every solved shift.