OxfordAQA International A Level Physics 3.5 Oscillations and Waves Exam Questions

About OxfordAQA International A Level Physics 3.5 Oscillations and Waves

Oscillations and waves is the second section of Unit 2 (PH02) on the OxfordAQA International AS course, and it is broad. It starts with mass-spring systems, pendulums, damping and resonance, then covers progressive, transverse and longitudinal waves, stationary waves, interference, diffraction and refraction. It finishes with quantum ideas: energy levels and line spectra, the photoelectric effect and wave-particle duality. Oscillations lead directly into simple harmonic motion in the A2 course. Examiners often test Young's double slit and diffraction grating calculations, explanations of how stationary waves form, total internal reflection in optical fibres, and why the photoelectric effect supports the photon model.

What you need to know

• Oscillating systems and resonance T = 2 pi square root of (m / k) for a mass-spring system and T = 2 pi square root of (L / g) for a simple pendulum. Damping removes energy. Resonance occurs when the driving frequency equals the natural frequency; heavier damping lowers and broadens the resonance peak.
• Progressive waves v = f lambda and f = 1 / T. Transverse waves (electromagnetic waves, waves on strings) can be polarised; longitudinal waves (sound, ultrasound) cannot. Phase difference is measured in degrees or radians.
• Stationary waves Formed by superposition of two waves of the same frequency travelling in opposite directions. Nodes have zero amplitude; antinodes have maximum amplitude. For a string fixed at both ends, the first harmonic has wavelength 2L.
• Interference and diffraction Coherent sources have the same frequency and a constant phase difference. For Young's double slits, fringe spacing w = lambda D / s. For a grating, d sin theta = n lambda. A single slit gives a wide central maximum that becomes wider as the slit narrows.
• Refraction n = c / cs and n1 sin theta1 = n2 sin theta2. Total internal reflection occurs when the angle of incidence exceeds the critical angle, sin thetac = n2 / n1. Optical fibres use a core with higher refractive index than the cladding.
• Energy levels and photoelectric effect Electrons move between discrete energy levels, emitting or absorbing photons with hf = E1 - E2, which gives line spectra. In the photoelectric effect, hf = phi + Ek max; below the threshold frequency no electrons are emitted whatever the intensity.
• Wave-particle duality Electron diffraction shows that particles behave as waves, with de Broglie wavelength lambda = h / mv. Faster electrons have shorter wavelengths, so the diffraction rings get smaller.

Common mistakes in 3.5 Oscillations and Waves

• Saying that brighter light gives photoelectrons more kinetic energy. Intensity changes the number emitted per second; the maximum kinetic energy depends only on frequency.
• Saying that a node is where the wave is not present. It is a point of permanent destructive interference where the amplitude is zero.
• Describing coherent sources as having the same phase. They need a constant phase difference and the same frequency.
• Forgetting to convert work function from eV to J (multiply by 1.60 x 10^-19) before using hf = phi + Ek max.
• Using n = 0 to n = 2 orders in a grating without checking that sin theta cannot exceed 1 when finding the maximum order.

Try three 3.5 Oscillations and Waves exam-style questions