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IBDP Physics- C.2 Wave model- IB Style Questions For SL Paper 1A -FA 2025

IBDP Physics SL Paper 1 -FA 2025- All Chapters

Question 

A sound wave in air is directed towards a water boundary, and part of it is refracted.

Three statements are made about the refracted wave compared to the incident wave.

I. The wavelength is different.

II. The amplitude is different.

III. The frequency is different.

Which of the statements are correct?

(A) I and II only
(B) I and III only
(C) II and III only
(D) I, II and III
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{A}} \)

When a wave crosses from one medium into another, its frequency remains unchanged because the frequency is determined by the source.

Therefore, statement III is incorrect.

The speed of sound is different in water compared with air. Using the wave equation

\(v=f\lambda\)

Since \(f\) remains constant but the wave speed changes, the wavelength must change.

Therefore, statement I is correct.

At the boundary, only part of the wave is transmitted and part may be reflected. The energy distribution changes, so the amplitude of the refracted wave is generally different from that of the incident wave.

Therefore, statement II is correct.

Thus, statements I and II only are correct.

Hence, the correct answer is \( \boxed{\mathrm{A}} \).

Question 

The graph shows how the displacement of a wave varies with the position \(x\) along the wave. The wave travels towards positive \(x\).

At the instant shown, which point along the wave has the maximum positive velocity?

(A) A
(B) B
(C) C
(D) D
▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathrm{B}} \)

For a progressive wave travelling in the positive \(x\)-direction, the displacement can be represented as

\(y=A\sin(kx-\omega t)\)

The particle velocity is

\(v_y=\frac{\partial y}{\partial t}=-\omega A\cos(kx-\omega t)\)

The magnitude of the velocity is greatest when the displacement is zero. At a zero crossing, the direction of the velocity depends on the direction of the wave and the slope of the displacement graph.

At B, the wave crosses the equilibrium position while the graph has a negative gradient. Since the wave travels towards positive \(x\), the displacement at this point is increasing with time, giving the maximum positive velocity.

Thus, point B has the maximum positive velocity.

Hence, the correct answer is \( \boxed{\mathrm{B}} \).

Question

A longitudinal wave is travelling through a medium. The variation with distance \(d\) of the displacement \(x\) of the particles in the medium at time \(t\) is shown.
Which point is at the centre of a compression?
(A) A
(B) B
(C) C
(D) D
▶️ Answer/Explanation
Detailed solution

For a longitudinal wave, a compression occurs where neighbouring particles are closest together.
This corresponds to a region where the displacement changes from positive to negative with distance (the curve has a steep negative slope), meaning particles on either side have moved towards the same region.

From the graph, point A is at the centre of a compression.

So:
A — compression
B — normal pressure point
C — rarefaction
D — normal pressure point

✅ Answer: (A)

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