Students will be expected to describe and explain interference produced…

The idea

Interference occurs when two waves of the same type overlap and their displacements combine. For sound, two loudspeakers driven by the same signal act as coherent sources; walking along a line parallel to them, you hear loud regions where path difference is a whole number of wavelengths and quiet regions where it is an odd number of half-wavelengths. For electromagnetic waves, laser light through two slits produces bright and dark fringes on a screen for the same reason. In both cases, the path difference between waves from the two sources determines whether they arrive in phase or out of phase. You should be able to describe the arrangement, explain the pattern using path difference, and calculate wavelength, source separation or fringe spacing using λ = ax/D for light or the equivalent path-difference reasoning for sound.

What earns the marks

• Describe a suitable arrangement for sound interference, such as two loudspeakers connected to one signal generator and a microphone or listener moved along a line.
• Describe a suitable arrangement for electromagnetic interference, such as laser light incident on two narrow slits with a screen to display fringes.
• Explain maxima in terms of path difference equal to a whole number of wavelengths, giving constructive superposition.
• Explain minima in terms of path difference equal to an odd number of half-wavelengths, giving destructive superposition.
• State that the sources must be coherent, meaning they maintain a constant phase difference, for a stable pattern to be observed.
• Use λ = ax/D for double-slit light interference, and apply path-difference reasoning to sound to find wavelength or source separation.

Common mistakes and how to put them right

• Treating loud and quiet regions as caused by changes in amplitude of the sources rather than by path difference.

  Put it right: Link each region to the phase relationship between the arriving waves.

• Assuming any two sound sources are coherent.

  Put it right: They must be driven by the same signal or otherwise maintain a constant phase difference.

• Using λ = ax/D with a as the distance from source to screen.

  Put it right: A is the separation of the two slits or sources and D is the distance to the screen or observation line.

Exam tips

• Name the wave type and the detector in your description, for example a microphone for sound or a screen for light, so the method is clear.
• State the path-difference condition for both maxima and minima; examiners look for both constructive and destructive cases.
• When calculating, keep D much larger than a where the approximation is used, and show the substitution before the answer.