How to play Harbour Gap
- Choose a test and read the wavelength, the gap, or the angle on the card.
- Use sin θ = λ ÷ a, with θ measured from straight ahead, and λ = v·T.
- Type your prediction and press Start. The wave gauge sweeps from straight ahead round to the wall.
- Compare the dashed orange lines with the calm directions, in the harbour and in the graph.
Diffraction
Waves that pass through a gap or round an obstacle don't carry on in a straight beam: they spread out into the space behind. This is diffraction. How much they spread depends on the gap compared with the wavelength. A gap many wavelengths wide lets through a fairly narrow beam; a gap about one wavelength wide spreads the waves out like ripples from a single point. Far behind a gap of width a, the waves are strongest straight ahead and fade to the sides, with calm directions where the waves from different parts of the gap cancel; the first is at sin θ = λ ÷ a. That is why you can hear someone round a corner (sound waves are about as long as a doorway is wide) but not see them (light waves are less than a thousandth of a millimetre long). Harbour engineers use the same rule to decide where boats will lie quietly behind a breakwater.
A focused learning model
The pattern is drawn as it would be far from the gap: wave height |sin β ÷ β| with β = π·a·sin θ ÷ λ, the same at every distance. Waves don't reflect off the wall or lose energy as they spread, and the sea floor is flat. The waves are drawn slowed down to one crest a second. Answers within 2% are right.