(j) use of a laser or light source to investigate characteristics of…

The idea

This practical skill requires you to set up a laser or other light source and use it to investigate the wave characteristics of light, specifically interference and diffraction. A typical method uses a laser directed at a single slit to produce a diffraction pattern, or at a double slit to produce interference fringes, with the pattern observed on a screen at a measured distance. You measure fringe spacing or minima positions and use the appropriate relationship, such as λ = ax/D for double-slit interference, to determine wavelength. You must control variables: slit separation, slit-to-screen distance and source wavelength. Safety matters: laser light must never be directed towards eyes, and beams should be kept below eye level.

What earns the marks

• Selects a monochromatic laser or filtered light source and directs the beam through a single or double slit onto a screen.
• Identifies and measures the relevant quantities, such as slit separation a, slit-to-screen distance D, and fringe spacing x or angular positions of minima.
• Uses the correct relationship, for example λ = ax/D for double-slit interference, or the diffraction grating equation d sin θ = nλ, to calculate wavelength.
• Describes the observed pattern: bright and dark fringes for interference, or a central maximum with subsidiary maxima for single-slit diffraction.
• Applies laser safety measures, such as avoiding direct eye exposure and keeping the beam path below eye level.

Common mistakes and how to put them right

• Confusing interference with diffraction.

  Put it right: Interference arises from superposition of waves from two or more coherent sources, while diffraction is the spreading of waves passing through an aperture or around an obstacle.

• Measuring fringe spacing across a single fringe rather than across many fringes and dividing, which increases percentage uncertainty.

  Put it right: Measure across several fringes and divide by the number of intervals.

• Forgetting to convert measurements to SI units, for example using millimetres for slit separation without converting to metres, which gives an incorrect wavelength.

Exam tips

• Always state the safety precaution explicitly when describing a laser experiment, as this is a standard assessed point.
• Show the rearrangement of the equation before substituting values, so the examiner can award method marks even if the final answer is wrong.
• When describing the pattern, use precise terms such as 'central maximum' and 'first-order minimum' rather than vague descriptions.