(ii) techniques and procedures used to determine the wavelength of light…

The idea

This statement introduces the practical techniques for measuring the wavelength of light by producing an interference pattern and analysing its geometry. A monochromatic source, such as a laser or a filtered lamp, illuminates either a double slit or a diffraction grating. The resulting pattern of bright and dark fringes is projected onto a screen, and measurements of fringe spacing or fringe positions are combined with the known slit or grating spacing and the screen distance. The wavelength is then calculated from the appropriate equation. The procedure requires care: the source must be coherent, the screen distance measured perpendicular to the source plane, and fringe spacings found by averaging over many fringes to reduce uncertainty. The two named techniques, double-slit and diffraction grating, are treated in the following statements.

What earns the marks

• Describes using a coherent monochromatic source to produce a stable interference pattern.
• Measures the distance from the source plane to the screen along the perpendicular axis.
• Measures fringe spacing or fringe positions on the screen, averaging over several fringes to reduce random uncertainty.
• Uses the known slit separation or grating spacing in the calculation of wavelength.
• Applies the appropriate equation, λ = ax/D for double slit or d sin θ = nλ for a diffraction grating.
• Identifies and controls sources of uncertainty, such as parallax in reading positions and the finite width of fringes.

Common mistakes and how to put them right

• Using a white-light source and expecting a stable, measurable fringe pattern.

  Put it right by using a monochromatic coherent source such as a laser.

• Measuring the distance between the source and the screen along a slanted line.

  Put it right by measuring D perpendicular to the source plane.

• Recording the position of a single fringe rather than averaging over many fringes.

  Put it right by measuring across several fringe intervals and dividing.

• Forgetting to convert slit separation or grating spacing from millimetres or lines per millimetre into metres before calculating.

Exam tips

• State the apparatus and the quantity measured at each stage; method marks depend on a clear sequence.
• Explain how averaging over many fringes reduces the percentage uncertainty in the fringe spacing.
• Keep the room darkened or use a laser so the fringes are clearly visible and positions can be read accurately.
• Quote the wavelength with a unit and a sensible number of significant figures consistent with the measurements.