Ripple tank: wide gap versus narrow gap
In a ripple tank with a straight-wave generator, a wide gap (b ≫ λ) lets the wavefronts pass through still almost straight, with slight curving only at the edges and a clear shadow region either side. A narrow gap (b ≈ λ) produces almost semicircular wavefronts spreading through the whole region beyond the barrier. In both cases the wavelength is unchanged.
spreading is significant when b ≈ λ; negligible when b ≫ λWhat this actually means
Two diagrams, one sentence each. Wide gap: straight wavefronts continue as a beam of about the gap width, curving only near the two edges. Narrow gap: circular wavefronts centred on the gap, filling the space behind.
The controlling quantity is the ratio b/λ, not b alone. A 3 cm gap is wide for light and narrow for sound. Always frame your answer as a comparison.
When you draw it, the spacing of the wavefronts after the gap must be identical to the spacing before. Drawing them closer together after the gap is the classic sketch error and it is heavily penalised.
The intensity falls as the wave spreads, because the same energy is now spread over a wider region. So the narrow gap gives more spreading but a dimmer, lower-amplitude wave.
In the tank you change the ratio either by moving the barriers closer together or by changing the frequency of the dipper, which changes λ since the water depth fixes the speed.
Drawing the diffracted wavefronts with a shorter wavelength or a different spacing after the gap.
Prove it — watch it be true
- In the ripple tank view, set the gap much wider than the wavelength and note the b/λ verdict and the near-straight wavefronts.
- Narrow the gap until b ≈ λ and watch the wavefronts become semicircular.
- Confirm the wavefront spacing readout is unchanged between the two cases.