DSE Physics Practice Paper: Waves, Sound And Optics

DSE Physics Practice Paper: Waves, Sound and Optics

2026-11-03

DSE Physics – Waves and Optics Practice

DSE Physics practice paper on waves, sound and optics. Twelve questions, 100 marks, 75 minutes. Covers wave speed and period, refraction and the critical angle.

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Paper details　DSE Physics, waves, sound, optics, refraction, interference, polarisation, 12 questions, 100 marks

Section A (Questions 1 to 4, 26 marks)

Read each question and write the correct answer with an explanation. Show working and units for any calculation.

#	Question	Answer and marking notes
1	Define a wave and give one example each of a transverse wave and a longitudinal wave, explaining the difference.	A wave is a disturbance that transfers energy through a medium or through space without transferring matter; the particles oscillate about fixed positions. In a transverse wave the oscillation is perpendicular to the direction of travel, for example a wave on a rope or a light wave. In a longitudinal wave the oscillation is parallel to the direction of travel, for example a sound wave, where air molecules vibrate along the direction of propagation.
2	A sound wave has frequency 50 Hz and wavelength 6.8 m. Find its speed and its period.	Speed = frequency x wavelength = 50 x 6.8 = 340 m/s. Period = 1 / frequency = 1 / 50 = 0.02 s, the time for one complete oscillation.
3	A ray of light passes from air into glass of refractive index 1.5 at an angle of incidence of 30 degrees. Find the angle of refraction to one decimal place.	By Snell law, sin r = sin 30 / 1.5 = 0.3333, so r = 19.5 degrees. Entering an optically denser medium the ray bends towards the normal, so the angle of refraction is smaller than the angle of incidence.
4	Find the critical angle for light travelling from this glass (refractive index 1.5) into air, and state what happens to a ray incident beyond that angle.	The critical angle satisfies sin c = 1 / n = 1 / 1.5 = 0.6667, giving c = 41.8 degrees. Beyond the critical angle no light is refracted into the air; the ray is totally internally reflected back into the glass, which is how optical fibres guide light.

Section B (Questions 5 to 8, 32 marks)

Answer the following questions with working, units and evidence drawn from the data given.

#	Question	Answer and marking notes
5	Using a ripple tank or two loudspeakers, state three conditions needed to observe a stable interference pattern.	The two sources must have the same frequency, a constant phase difference and comparable amplitudes at the region of overlap. In practice the sources are driven by one oscillator so that their phase difference stays fixed; if the amplitudes differ greatly the fringes are too faint to see.
6	In a double slit experiment the slit separation is 0.50 mm, the screen distance is 2.0 m and the wavelength is 600 nm. Find the fringe spacing.	Fringe spacing = wavelength x screen distance / slit separation = 600 x 10^-9 x 2.0 / (0.50 x 10^-3) = 2.4 mm. Because the wavelength is far smaller than the slit separation the fringes are evenly spaced, so the pattern can be used to measure an unknown wavelength.
7	A diffraction grating has 500 lines per millimetre. Light of wavelength 600 nm falls normally on it. Find the angle of the first order maximum and the highest order visible.	The grating spacing is d = 1 mm / 500 = 2.0 micrometre. From d sin theta = n x wavelength, the first order gives sin theta = 0.300, so theta = 17.5 degrees. The highest order needs n <= d / wavelength = 3.33, so the third order is the highest; a fourth order would need sin theta greater than 1.
8	A convex lens of focal length 10 cm has an object placed 15 cm in front of it. Find the image distance and magnification, and describe the image.	From 1 / f = 1 / u + 1 / v, 1 / v = 1 / 10 – 1 / 15 = 1 / 30, so v = 29 cm. Magnification = -v / u = -2, so the image is twice the size and inverted. As v is positive the image is real and can be caught on a screen: inverted, magnified and real.

Section C (Questions 9 to 12, 42 marks)

Write fuller answers using correct subject terms and support them with examples from Hong Kong or daily life.

#	Question	Answer and marking notes
9	Explain, in terms of the change in wave speed, why a ray of light bends towards the normal on entering water, and why the frequency does not change.	The speed of light in a medium is c / n, so it slows down on entering water. The lower part of each wavefront enters the water first and slows while the upper part is still travelling faster in air, so the wavefront turns and the ray bends towards the normal. The frequency is set by the source and the number of waves arriving per second at the boundary does not change, so the frequency stays the same while the wavelength shortens.
10	Explain how polarisation shows that light is a transverse wave, and give one everyday use of a polarising filter.	A polarising filter transmits only the component of the electric field oscillating along one direction. Turning a second filter changes the transmitted intensity, falling to almost zero when the axes are at right angles. Only transverse waves show this directional selection, since a longitudinal wave oscillates along the direction of travel. Everyday uses include sunglasses and camera filters that cut glare reflected from water and roads.
11	A string of length 1.2 m and linear density 2.0 x 10^-3 kg/m is fixed at both ends and under a tension of 45 N. Find the wave speed, the fundamental frequency and the third harmonic.	Speed = square root of (tension / linear density) = square root of (45 / 2.0 x 10^-3) = 150 m/s. For a string fixed at both ends the fundamental is f1 = v / 2L = 150 / 2.4 = 62.5 Hz. The third harmonic is 3 f1 = 187.5 Hz, with three antinodes and a wavelength of 2L / 3.
12	The sound intensity at a point is 1.0 x 10^-4 W/m2. Find the sound intensity level, and the new level when a second identical source is added.	Sound intensity level = 10 log (I / 10^-12) = 10 log (1.0 x 10^-4 / 10^-12) = 10 x 8 = 80 dB. A second identical source doubles the intensity, adding 10 log 2, about 3 dB, giving roughly 83 dB. Doubling the intensity therefore raises the level by only about 3 dB, which the ear hears as slightly louder.

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