A-Level · 26 September 2026 · 6 min read
Diffraction and interference: worked A-Level Physics questions
The difficult part of a waves question is often deciding what the diagram represents. A broad central diffraction maximum, equally spaced two-slit fringes and sharp grating orders are related wave effects, but they are not interchangeable diagrams. Start by naming the apparatus and what the measured distance or angle connects.
These original examples practise wave reasoning relevant to Edexcel IAS Unit 2. They use idealised arrangements and state the assumptions needed. The associated notes explain the underlying concepts; this guide focuses on the decisions, calculations and checks that turn those concepts into answers.
Translate path difference into a phase argument
For two sources initially in phase, a path difference of one whole wavelength preserves the phase relationship at the observation point, giving constructive interference. A path difference of half an odd number of wavelengths gives opposite phases and destructive interference. Include the source phase condition: a diagram with an imposed phase shift needs that shift included.
Coherent sources maintain a constant phase difference and have the same frequency. They do not have to be in phase everywhere. If a question asks why two unrelated lamps do not produce a stable fringe pattern, saying “the light is not strong enough” misses the issue: the phase relationship is not maintained in the required way.
Worked example: double-slit fringe spacing
Light of wavelength 600 nm passes through slits separated by 0.50 mm. A screen is 2.0 m away. Using the small-angle relationship, w = λD/s = (600 × 10⁻⁹ × 2.0)/(0.50 × 10⁻³) = 2.4 × 10⁻³ m, or 2.4 mm. The separation in the denominator is between the slits, not the width of one slit.
If measuring ten fringe spacings, the expected distance is 24 mm. Eleven adjacent bright-fringe centres define ten intervals, which is a common counting trap. Measuring several intervals and dividing can reduce the relative effect of endpoint reading uncertainty, provided you identify the fringes consistently and keep the setup stable.
Change one condition and predict the pattern
Keeping wavelength and slit separation fixed, moving the screen from 2.0 m to 3.0 m increases spacing to 3.6 mm. Keeping the screen fixed but doubling slit separation reduces spacing to 1.2 mm. Longer wavelength produces wider spacing. Predict these changes before entering numbers so a mistaken rearrangement becomes easier to spot.
The relation assumes the fringe angles are small and the screen distance is large compared with the slit separation. If the question provides geometry outside that approximation, use the required trigonometric reasoning instead of treating w = λD/s as exact in every arrangement. State the approximation when asked to justify the method.
Worked example: convert a grating specification
Light is incident normally on a grating with 500 lines per millimetre. That is 5.0 × 10⁵ lines per metre, so d = 1/(5.0 × 10⁵) = 2.0 × 10⁻⁶ m. For 600 nm light in first order, sin θ = λ/d = 0.30, giving θ = 17.5°. The angle is measured from the grating normal, which is the central-maximum direction for normal incidence, not from the grating surface.
For second order, sin θ = 0.60 and θ = 36.9°. The second-order angle is not exactly twice the first-order angle because sine is not proportional to angle over this range. Use the grating equation again rather than doubling the numerical answer. Check that the calculator is in the angle mode you intend to use.
Worked example: decide whether an order can exist
For the same grating and wavelength, nλ/d must not exceed 1. The ratio d/λ is 3.33, so the largest integer order permitted is 3. Fourth order would require sin θ = 1.20, which has no physical solution. The central order is n = 0, with nonzero orders appearing on both sides in the ideal symmetrical arrangement.
If the number of lines per millimetre increases, grating spacing decreases. The angular separation of allowed orders increases, but fewer orders may be possible. “More lines gives more maxima” therefore confuses two different ideas. Explain the consequence through both the equation and the sine limit.
Distinguish diffraction width from interference spacing
Narrowing a single slit increases diffraction spreading for a fixed wavelength. In a double-slit arrangement, changing each slit’s width affects the diffraction envelope, while changing their separation affects fringe spacing. Questions may combine these effects, so label width and separation with different symbols rather than calling both “the gap”.
Finish your practice by drawing the three arrangements and annotating the quantity measured in each. Then solve one numerical question without the diagram labels supplied. Check wavelength conversion, interval counting, reciprocal line density, angle reference and possible order. Those checks are more useful than memorising three unlabeled formulas.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
What is the difference between diffraction and interference?
Diffraction describes spreading associated with an aperture or obstacle. Interference describes the combination of waves and the resulting reinforcement or cancellation. Diffraction patterns can themselves be explained through interference, so the ideas are connected. In questions, identify the apparatus and the measured feature before choosing an equation.
How do I convert lines per millimetre to grating spacing?
Multiply the line density by 1000 to express it per metre, then take its reciprocal. For 500 lines per millimetre, the density is 500,000 m⁻¹ and spacing is 2.0 × 10⁻⁶ m. Do not substitute line density directly for d in d sin θ = nλ.
Do coherent waves have to be in phase?
No. Coherent waves have the same frequency and a constant phase difference. That difference may be zero or another fixed value. When deciding where reinforcement occurs, include both the starting phase difference and the extra phase caused by unequal path lengths.
How do I find the highest diffraction-grating order?
For normal incidence, use nλ/d ≤ 1 because sin θ cannot exceed 1. Find the greatest integer n allowed by that inequality, then check the physical arrangement. A calculated sine greater than 1 means that proposed order cannot occur, rather than indicating a calculator fault.
How can double-slit fringes be made wider?
Within the small-angle model, increase wavelength, increase screen distance or reduce slit separation. Change one quantity at a time and state which are fixed. Slit separation and the width of an individual slit affect different features of the overall pattern.