Required practical 2: Investigation of interference effects to include the…

The idea

In this required practical you use a monochromatic light source, such as a laser, to produce interference patterns. For Young's slit experiment, light passes through two narrow slits and forms equally spaced fringes on a screen; you measure the fringe spacing w, slit separation s and slit-to-screen distance D, then use w = λD / s to determine wavelength. For a diffraction grating, light passes through many slits and produces sharp maxima at angles θ given by d sin θ = nλ, where d is the grating spacing and n is the order. You should record multiple orders, measure angles accurately and evaluate uncertainties. The practical develops skills in setting up optical apparatus, measuring small distances and using trigonometric relationships.

What earns the marks

• Set up a laser and double slit so that a clear interference pattern is visible on a screen, ensuring the slits are vertical and the screen is perpendicular to the beam.
• Measure the distance D from the slits to the screen and the separation s between the slits, using a ruler or travelling microscope as appropriate.
• Measure the fringe spacing w by recording the distance across several fringes and dividing by the number of fringe intervals to reduce percentage uncertainty.
• Use w = λD / s to calculate the wavelength of the light, and compare with the expected value or use it to find an unknown slit separation.
• For a diffraction grating, measure the angle θ for each visible order n on both sides of the central maximum and use the mean angle in d sin θ = nλ.
• Determine the grating spacing d from the number of lines per metre, for example d = 1 / N where N is the number of lines per metre.
• Identify and control sources of uncertainty, such as measuring across multiple fringes, avoiding parallax when reading angles, and keeping the room dark to improve contrast.

Common mistakes and how to put them right

• Using the distance from the first to the last fringe as the fringe spacing.

  Put it right: Divide the total distance by the number of fringe intervals, not by the number of fringes counted.

• Confusing slit separation s with slit width.

  Put it right: s is the centre-to-centre distance between the two slits.

• Forgetting to convert lines per millimetre to metres when finding d.

  Put it right: Convert to lines per metre first, then use d = 1 / N.

Exam tips

• Show the equation you are using, substitute values with consistent units, and state the unit of the final answer; for example, convert all lengths to metres before calculating λ.
• When describing the method, include how you improve accuracy: measure across many fringes, use a darkened room, and take repeat readings of angles on both sides of the centre.
• For diffraction grating questions, always identify n as the order number and use the angle from the central maximum to the nth-order maximum, not the angle between adjacent maxima.