Pearson International A Level Physics Core Practical 6: Determine light wavelength using a diffraction grating

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Use diffraction geometry and a known grating spacing to determine light wavelength.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use laser or monochromatic source, diffraction grating, screen, metre rule, set square and secure optical stands. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Mount the grating normal to the beam and measure grating-to-screen distance.
2. Mark symmetric diffraction orders and measure their separation over the widest safe order range.
3. Calculate each angle from the measured geometry rather than using a small-angle shortcut unless justified.
4. Apply d sin theta = n lambda and average consistent order estimates.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: diffraction order or grating spacing if comparing gratings.
• Dependent variable: diffraction angle or fringe position.
• Important controls: wavelength source, alignment, screen distance and order identification.