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A-level Physics required practicals

Young's slits and diffraction-grating interference

AQA 3.3.2.1 · RP2

A-level Physics (7408) · Required practical 2 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7408 · registry checked 2026-07-11How this checking works

Determine the wavelength of monochromatic light from two-source fringe spacing and from diffraction-grating angles.

Apparatus

  • Low-power visible laser with a labelled wavelength
  • Young double slit with known slit separation
  • Diffraction grating with known line density
  • Screen, metre rules and stands
  • Protractor or angle scale for symmetric diffraction orders

Apparatus & techniques (AT)

AT a · analogue measurement

Metre rules and an angular scale provide the screen-distance, fringe-span and diffraction-angle measurements.

AT j · laser/light investigation

A low-power laser illuminates the double slit and grating so interference and diffraction patterns can be measured safely.

Safety

Hazard

Direct or specularly reflected laser light can damage the eye.

Control

Use a low-power classroom laser below eye level, terminate the beam on a screen, remove reflective jewellery and never look along the beam.

Hazard

A darkened working area increases trip risk.

Control

Keep enough room lighting to move safely and keep stands and bags out of walkways.

Method

  1. 1Fix the laser so its beam is level and normal to the slit or grating; mark the central maximum without looking into the beam.
  2. 2For Young's slits, place the screen far enough away for a measurable pattern, measure the distance D normal to the slit plane and span several fringe intervals on the screen.
  3. 3Divide the measured span by the exact number of fringe intervals, then repeat at several screen distances if a graph method is used.
  4. 4Replace the slits with the grating, keep the beam normal to it and mark matching diffraction orders on both sides of the centre.
  5. 5Measure left and right angles for several orders — directly, or from tan theta = y/D using the order displacement y and the grating-to-screen distance D — average their magnitudes and record which orders are sharp enough to locate reliably.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: select a screen distance, fringe span and accessible diffraction orders that give useful percentage uncertainties.
  • CPAC 3: Editorial focus: control the laser beam path and prevent direct or reflected exposure to eyes.
  • CPAC 4: Editorial focus: record symmetric angular readings and clearly distinguish fringes, intervals and order number.
  • CPAC 5: Editorial focus: process both geometries, use a gradient where appropriate and compare wavelength estimates with uncertainty.

Variables

Independent

Screen distance D for Young's slits, or diffraction order n for the grating

Dependent

Fringe spacing w or diffraction angle theta

Control

  • Laser wavelength and slit/grating spacing
  • Normal incidence and the central-reference position
  • The same fringe/order definition and measurement geometry

Results & processing

  • For Young's slits, calculate wavelength from lambda = ws/D or obtain lambda/s from the gradient of w against D.
  • For the grating, calculate d from the line density and use n lambda = d sin(theta); a graph of sin(theta) against n has gradient lambda/d.
  • Compare the two wavelength estimates with their uncertainties and with the laser label without treating agreement to every displayed digit as meaningful.

Analysis skills

  • Apply lambda = ws/D and n lambda = d sin(theta) with all distances in consistent SI units.
  • Find lambda from the gradient of w against D or sin(theta) against n, including acceptable max/min gradients.

Uncertainty

Sources

  • Finite fringe width and uncertainty in locating fringe centres
  • Angular scale resolution, central-zero offset and imperfect normal incidence
  • Uncertainty in slit separation, grating line density and screen distance

Calculations

  • For lambda = xs/(ND), where x spans N exact intervals, add the percentage uncertainties in x, s and D.
  • For a graph result, use max/min acceptable gradients rather than propagating each plotted point into a single nominal gradient.

Interpretation

  • Left/right disagreement suggests alignment or zero error and should not be hidden by quoting only their mean.
  • A wavelength is consistent with a reference when the difference is comparable with the combined uncertainty, not merely because rounded values look similar.

Exam angles

  • Explain exactly why spanning many fringes lowers percentage uncertainty.
  • Diagnose impractical placement or alignment from a plan or geometry diagram.
  • Use grating evidence in an unfamiliar context and reserve resolving-power claims for cases where the required information is supplied.

Where students lose marks

Putting the screen too close or measuring only one narrow fringe interval.

Fix: Use a large practical D and span several complete intervals so the absolute reading uncertainty is a smaller fraction of the distance.

  • Examiner report: P3-17 · PDF p. 3

Claiming that an apparatus change reduces uncertainty without naming the reading whose percentage uncertainty falls.

Fix: State the larger measured span or angle, keep the absolute scale uncertainty explicit, and compare the resulting percentages.

Measuring every grating angle from a table edge rather than the central maximum.

Fix: Measure symmetric orders from the centre and average left/right magnitudes to reduce angular-zero effects.

Improve the method

  • Measure across many fringe intervals and repeat from two independently marked endpoints.
  • Use several diffraction orders on both sides of the central maximum and fit sin(theta) against order.
  • Clamp the laser, slit/grating and screen so their centres share one horizontal line and check normal incidence before readings.

Source references

  • Specification: PSpec 3.3.2.1 · PDF p. 20

Try it — exam-style

Hard
ORIGINAL

A student measures x = (24.0 +/- 0.5) mm across 8 fringe intervals. The slit separation is s = (0.250 +/- 0.010) mm and D = (1.800 +/- 0.010) m. Calculate lambda = xs/(8D) and its absolute uncertainty using added percentage uncertainties.

[5 marks]

Total for this question: 5

Medium
ORIGINAL

Explain why a student measures the first-order grating maximum on both sides of the central maximum rather than using one angle only.

[2 marks]

Total for this question: 2

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

AQA 7408 specification coverage

Interference, diffraction and every linked 7408 specification point.

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