3.3.2.1
Required practical: Young's double slit and diffraction gratings
Interference & Diffraction — AQA A-Level Physics
Young's double slit: measuring wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m).
- Independent variable: slit-to-screen distance D. Dependent variable: fringe width w. Controls: wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)., slit separation s.
- Set up laser, single slit, double slit and screen. Darken the room.
- Measure across many fringes with vernier callipers and divide by the number of fringes to find w.
- Vary D from 0.5 m to 1.5 m in 0.1 m steps. Take 3 readings of w at each D.
- Plot w (y) against D (x). $Gradient = \lambda / s. So \lambda = gradient x s$.
Diffraction gratingAn optical component with many equally spaced parallel slits that produces sharp interference maxima at specific angles.: measuring wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m).
- Set up laser and diffraction gratingAn optical component with many equally spaced parallel slits that produces sharp interference maxima at specific angles. with screen at $D = 1.0 m$.
- Use a set square to ensure the beam passes through the grating at normal incidence.
- Measure the distance h from the central maximum to the nth order maximum on each side. Take the mean.
- Calculate the angle: $\theta = \tan^{-1}(h / D)$.
- Calculate wavelength from $\lambda = d \sin \theta / n$.
- Repeat for different orders and different gratings. Calculate a mean wavelength.
Reducing uncertainties
- Use a set square to ensure normal incidence (reduces systematic errorAn error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements.).
- A grating with more lines per mm gives larger h values, reducing percentage uncertainty.
- Measure from the centre of each bright fringe.
- Measure across all visible fringes and divide by the number of fringes.
- Increase D to increase fringe separation (though intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. may decrease).
- Conduct the experiment in a darkened room for clearer fringes.