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

IBDP Physics HL 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 critical angle for light passing from a medium into air is \(30^\circ\).

What is the approximate value for the speed of light in the medium?

(A) \(0.5\times10^8\,\mathrm{m\,s^{-1}}\)
(B) \(1.5\times10^8\,\mathrm{m\,s^{-1}}\)
(C) \(2.0\times10^8\,\mathrm{m\,s^{-1}}\)
(D) \(2.6\times10^8\,\mathrm{m\,s^{-1}}\)
▶️ Answer/Explanation

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

For light travelling from a medium into air, the critical angle is related to the refractive index by

\(\sin C=\dfrac{n_{\mathrm{air}}}{n_{\mathrm{medium}}}\)

Taking \(n_{\mathrm{air}}\approx1\),

\(n_{\mathrm{medium}}=\dfrac{1}{\sin C}\)

For \(C=30^\circ\),

\(n_{\mathrm{medium}}=\dfrac{1}{\sin30^\circ}=2\)

The refractive index is related to the speed of light by

\(n=\dfrac{c}{v}\)

Therefore,

\(v=\dfrac{c}{n}\)

\(v=\dfrac{3.0\times10^8}{2}\)

\(v=1.5\times10^8\,\mathrm{m\,s^{-1}}\)

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?
▶️ Answer/Explanation
Detailed solution

In a longitudinal wave, compressions occur where particles of the medium are closest together, corresponding to regions of maximum pressure and density.

On a displacement–distance graph, the centre of a compression is located where the gradient of the displacement is most negative, indicating particles on either side are moving towards each other.

From the diagram:
Point A corresponds to a compression,
Point B is a normal pressure point,
Point C corresponds to a rarefaction,
Point D is another normal pressure point.

✅ Answer: (A)

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