Diffraction of waves through a narrowing gap

Steps:

• Plane waves of constant frequency approach the barrier uniformly.
• Waves passing through the gap diffract, spreading out due to wave nature.
• Smaller gap narrows the wavefront, increasing diffraction angle and post-gap spreading.
• Correct diagram contrasts minimal spreading for wide gap with maximal for narrow gap.

Why D is correct:

• Huygens' principle states each point on wavefront acts as source; smaller gap limits sources, maximizing diffraction spread.

Why the others are wrong:

• A: Depicts reflection waves bouncing back, ignoring diffraction.
• B: Shows unchanged straight waves, contradicting increased spreading.
• C: Illustrates focusing or convergence, opposite of diffraction effect.

Final answer: D