Paper 2 · Waves and Interference
Why does pattern A (Fig. 4.2) correspond to the narrower gap?
AIn A the wavefronts emerge as almost complete semicircles, showing strong diffraction from a gap comparable in width to the wavelength; in B the wavefronts stay largely straight, showing weaker diffraction from a wider gap.
BIn A the wavefronts appear to travel measurably faster after emerging from the gap than they did on the approach side, and this apparent speed increase is assumed here to occur only when the gap the wave passes through is very narrow.
CIn A the wavelength of the wave increases after passing through the gap, a change that is unique to narrow gaps.
DIn A the wave amplitude is measurably higher after the gap than in B, which indicates the narrower gap width.
Explanation: In A the wavefronts spread as almost complete semicircles, strong diffraction which occurs when the gap width is comparable to the wavelength. In B the wavefronts stay largely straight, weak diffraction typical of a gap much wider than the wavelength.
Derived from ZIMSEC Physics Paper 2, November 2002, Q4