CCEA A Level Physics
CCEA A Level Physics 2.4 Superposition, Interference and Diffraction Exam Questions
Free CCEA A Level Physics revision for 2.4 Superposition, Interference and Diffraction: what you need to know, common mistakes and exam-style questions with answers. Covers superposition, standing waves, the resonance tube, coherence, young's slits, single slit diffraction and diffraction gratings.
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About CCEA A Level Physics 2.4 Superposition, Interference and Diffraction
This section of Unit AS 2: Waves, Photons and Astronomy deals with what happens when waves meet. You need the principle of superposition, how standing waves form and the modes of vibration on strings and in air columns, and the resonance tube method for the speed of sound. Interference needs coherent sources, so you must explain coherence and path difference, then use Young's double-slit experiment to measure wavelength. Single slit diffraction and the diffraction grating complete the topic. Typical exam questions compare standing and progressive waves, find the speed of sound from two resonance lengths, and use d sin θ = nλ to find a wavelength or the highest order.
What you need to know
- Superposition When two waves meet, the resultant displacement at any point is the vector sum of the individual displacements.
- Standing waves Formed by superposition of two waves of the same frequency and amplitude travelling in opposite directions. Nodes have zero amplitude; antinodes have maximum amplitude; adjacent nodes are λ/2 apart. Energy is not transferred along a standing wave.
- Strings and air columns A string fixed at both ends has nodes at the ends, so the fundamental has λ = 2L. A pipe closed at one end has a node at the closed end and an antinode at the open end, so the fundamental has λ = 4L (ignoring end correction).
- Resonance tube Vary the length of air above water in a tube held over a tuning fork until resonance. The first two resonance lengths differ by λ/2, so λ = 2(l2 - l1) and v = fλ, which removes the end correction.
- Interference and Young's slits Coherent sources have a constant phase difference and the same frequency. Bright fringes occur where path difference = nλ and dark fringes where it is (n + 1/2)λ. Fringe spacing y = λD/a, where a is slit separation and D the slit-to-screen distance.
- Diffraction A wave spreads out through a gap; the effect is greatest when the gap is similar in size to the wavelength. A single slit gives a wide central maximum. A diffraction grating gives sharp maxima at d sin θ = nλ, where d is the line spacing.
Common mistakes in 2.4 Superposition, Interference and Diffraction
- Saying any two sources interfere; they must be coherent for a stable pattern.
- Using a node-to-node distance of λ instead of λ/2.
- Forgetting to convert lines per mm to slit spacing d in metres, e.g. 300 lines per mm gives d = 3.33 x 10^-6 m.
- Rounding the highest order up; the maximum order is the whole number below d/λ.
- Describing the central maximum of a single slit as the same width as the others; it is twice as wide.
Try three 2.4 Superposition, Interference and Diffraction exam-style questions
1. Light of wavelength 600 nm falls on a grating with 300 lines per mm. Calculate the angle of the first order maximum and the highest order visible. (4 marks)
Show the mark scheme answer
d = 1/300 000 = 3.33 x 10^-6 m; sin θ = λ/d = 600 x 10^-9 / 3.33 x 10^-6 = 0.18, θ = 10.4°; n(max) = d/λ = 5.55; highest order = 5.
2. In a Young's slits experiment the slit separation is 0.50 mm and the screen is 2.0 m away. Light of wavelength 600 nm is used. Calculate the fringe spacing. (2 marks)
Show the mark scheme answer
y = λD/a = 600 x 10^-9 x 2.0 / 0.50 x 10^-3; = 2.4 x 10^-3 m (2.4 mm).
3. State two differences between a standing wave and a progressive wave. (2 marks)
Show the mark scheme answer
A standing wave does not transfer energy, a progressive wave does; in a standing wave amplitude varies with position from zero at nodes to maximum at antinodes whereas in a progressive wave all points have the same amplitude; (or) points between adjacent nodes oscillate in phase in a standing wave.
2.4 Superposition, Interference and Diffraction FAQs
What does CCEA A Level Physics 2.4 cover?
Superposition, standing waves on strings and in air columns, measuring the speed of sound with a resonance tube, interference and coherence, Young's slits, single slit diffraction and the diffraction grating.
Which unit is 2.4 examined in?
It is in Unit AS 2: Waves, Photons and Astronomy.
How do I avoid the end correction in the resonance tube experiment?
Measure the first and second resonance lengths with the same tuning fork. Their difference is exactly half a wavelength, so the unknown end correction cancels out.
