Diffraction pattern through a gap

Steps:

• Observe incoming plane waves approaching the gap between barriers.
• Apply Huygens' principle: each point on the wavefront acts as a source of secondary wavelets.
• Expect waves to spread out circularly after the gap if gap width ≈ wavelength.
• Select diagram matching semicircular wavefronts beyond the gap.

Why B is correct:

• It shows waves diffracting and spreading into a semicircular pattern, consistent with Huygens' principle for obstacles comparable to wavelength.

Why the others are wrong:

• A: Depicts straight-line propagation without bending, violating diffraction.
• C: Illustrates incorrect focusing or convergence instead of spreading.
• D: Shows reflection back toward the source, not transmission through the gap.

Final answer: B