Fig. 10.1 shows the entrance to a harbour. Sea waves approach the harbour entrance from the open sea. The harbour wall has a gap of width d. (a) The wavelen...

(a) When the wavelength is much smaller than the gap width d:

• The waves pass through the gap and continue mostly as plane (straight) wavefronts, with a shadow zone behind each section of wall where no waves reach. [1]
• Only slight curving of the wavefronts occurs at the very edges of the gap. [1]

Diffraction is negligible when the gap is much wider than the wavelength. The waves travel through almost as if the walls were not there, and the regions directly behind the harbour walls receive little wave energy.

(b) When the wavelength is approximately equal to d:

• The waves spread out much more after passing through the gap. [1]
• The wavefronts become semicircular, reaching areas behind the harbour walls that were previously in the shadow zone. [1]

Maximum diffraction occurs when the gap width is comparable to (or smaller than) the wavelength. The gap effectively acts as a new point source, radiating circular wavefronts into the harbour.

(c) The wave effect is called diffraction. [1]

Diffraction is the spreading of waves as they pass through a gap or around an obstacle. It is a property of all waves.

(d)

• Waves diffract through the harbour entrance and spread into the enclosed area. [1]
• Even though the entrance is narrow, diffraction causes the waves to spread out and reach boats moored deep inside the harbour. [1]

In fact, a narrower entrance (closer to the wavelength of the sea waves) produces stronger diffraction, so the waves spread more effectively into the harbour interior. This is why boats far from the entrance can still bob up and down on the diffracted waves.