Required Practicals / OCR A / Practical 4
4 AS PAG 4

Determining wavelength using a diffraction grating (PAG 4)

Determine the wavelength of monochromatic light using a diffraction grating; also investigate Young's double slit fringes.

Apparatus

  • Laser or bright monochromatic source
  • Diffraction grating (known lines per mm)
  • Double slit card
  • Screen, metre rule
  • Protractor or spectrometer

Safety

  • Never look into the laser beam directly.
  • Display a laser warning sign and ensure the beam cannot reach anyone's eyes.

Method

  1. Diffraction grating: direct laser through grating onto screen. Measure distance D to screen and distances $y_n$ to each order n.
  2. Calculate $\tan\theta = y_n/D$; apply $d\sin\theta = n\lambda$ for each order.
  3. Young's slits: direct laser through double slit. Measure fringe spacing w and slit-screen distance D.
  4. Calculate $\lambda = aw/D$ where a is slit separation.
  5. Compare wavelengths obtained by both methods.

Key Variables

Independent Order n (grating) or distance D (Young's)
Dependent Angle theta (grating) or fringe spacing w (Young's)
Controlled Same laser throughout; Grating/slit properties constant

Analysis and Results

  • Grating: $d\sin\theta = n\lambda$. Plot $\sin\theta$ vs n: gradient $= \lambda/d$.
  • Young's slits: $\lambda = aw/D$. Measure w across many fringes to reduce uncertainty.
  • Both methods should yield consistent values of $\lambda$.

Common Errors

  • Using $\theta = y/D$ (small angle approximation) for large angles (grating).
  • Measuring fringe spacing across too few fringes in Young's slits.
  • Confusing $d$ (slit spacing) with $N$ (lines per mm).

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 3 marks

A grating with 500 lines mm$^{-1}$ gives a second-order maximum at 38.5 degrees. Calculate the wavelength.

Q2 2 marks

Explain why using the second-order maximum rather than the first gives a more precise measurement of wavelength.