Required Practicals / Edexcel / Practical 6
6 AS CP6

Determination of wavelength using a diffraction grating (CP6)

Determine the wavelength of monochromatic light using a diffraction grating.

Apparatus

  • Laser or monochromatic light source with filter
  • Diffraction grating (known line spacing d)
  • Screen and ruler (or spectrometer with protractor)
  • Metre rule

Safety

  • Never look directly into the laser. Post a warning sign on the door.
  • Ensure no specular reflections from the grating or screen surface can leave the lab.

Method

  1. Direct the laser through the diffraction grating onto a screen ~1.0 m away.
  2. Measure the distance D from grating to screen and the distances $y_n$ from the central maximum to each order n.
  3. Calculate $\tan\theta_n = y_n/D$, hence $\theta_n = \arctan(y_n/D)$.
  4. Apply $d\sin\theta_n = n\lambda$ to find $\lambda$ for each order.
  5. Average the values of $\lambda$ obtained from orders n = 1 and n = 2 (and $n = -1$, $-2$ if visible).

Key Variables

Independent Order number n
Dependent Diffraction angle theta
Controlled Line spacing d (same grating); Wavelength (same laser)

Analysis and Results

  • $d\sin\theta = n\lambda$. Plot $\sin\theta$ vs n: straight line through origin, gradient $= \lambda/d$.
  • Check that $\sin\theta < 1$ for all measured orders (beyond this, no diffraction maximum exists).
  • A spectrometer gives more precise angle measurements than a ruler and screen.

Common Errors

  • Measuring the angle from the wrong side of the central maximum.
  • Using $\theta = y/D$ instead of $\tan\theta = y/D$ (significant error for large angles).
  • Confusing d (slit spacing in metres) with N (lines per mm or per metre).

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

Q1 3 marks

A laser passes through a grating with 600 lines mm$^{-1}$. The first-order maximum appears at 21.1 degrees. Calculate the wavelength.

Q2 2 marks

Explain why a grating with 600 lines mm$^{-1}$ produces sharper maxima than one with 100 lines mm$^{-1}$.