Required Practicals / AQA / Practical 2
2 AS 3.4.2.2

Interference: Young's double slit and diffraction grating

Determine the wavelength of monochromatic light using Young's double slit experiment and a diffraction grating.

Apparatus

  • Laser (or bright monochromatic lamp with filter)
  • Double slit card (slit separation a approx 0.5 mm)
  • Diffraction grating (e.g. 300 lines/mm)
  • Screen or wall and metre rule
  • Travelling microscope or ruler for fringe spacing

Safety

  • Never look directly into the laser beam or its specular reflection.
  • Place a beam stop behind the screen; do not allow the beam to leave the room.

Method

  1. Young's slits: shine the laser through the double slit onto a screen ~1.0 m away. Measure the fringe spacing w by spanning at least 5 fringes and dividing.
  2. Measure the slit-to-screen distance D. Obtain slit separation a from the manufacturer or a travelling microscope.
  3. Calculate wavelength: $\lambda = aw/D$. Repeat for three values of D.
  4. Diffraction grating: shine the laser through the grating. Mark the positions of the central maximum and first/second-order maxima on a screen.
  5. Measure the angle theta for each order n and apply $d\sin\theta = n\lambda$ where $d = 1/N$ (N = lines per metre).

Key Variables

Independent Slit-to-screen distance D (Young's) or order number n (grating)
Dependent Fringe spacing w (Young's) or diffraction angle theta (grating)
Controlled Slit separation a; Wavelength of light (same laser); Grating line spacing d

Analysis and Results

  • Young's slits: $\lambda = aw/D$. Plot w against D: straight line through origin, gradient $= \lambda/a$.
  • Diffraction grating: $d\sin\theta = n\lambda$. Plot $\sin\theta$ against n: straight line through origin, gradient $= \lambda/d$.
  • Diffraction grating gives sharper, brighter maxima than Young's slits, allowing more precise angle measurement.

Common Errors

  • Measuring fringe spacing across only one or two fringes, increasing percentage uncertainty.
  • Measuring D from the laser rather than from the double slit.
  • Misidentifying orders in the diffraction grating (central maximum is n = 0, not n = 1).
  • Using non-monochromatic light, which produces overlapping coloured fringes.

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

Q1 3 marks

Monochromatic light passes through a double slit of separation 0.40 mm. The fringe spacing on a screen 1.2 m away is 1.5 mm. Calculate the wavelength of the light.

Q2 3 marks

A diffraction grating has 300 lines per mm. Light of wavelength 589 nm is incident normally. Calculate the angle of the second-order maximum.

Q3 2 marks

State one advantage of using a diffraction grating rather than a double slit to measure the wavelength of light.