Plane waves through a narrow gap emerge as circular waves. This phenomenon is known as

Step 1: Analyze the question Plane waves pass through a narrow gap and come out as circular waves. We need the name of this phenomenon.

Step 2: Explain the concept When a wave meets an obstacle or passes through an opening, it spreads out into the region behind it. This is called diffraction. The effect is strongest when the gap width is about the same size as, or smaller than, the wavelength of the wave.

Step 3: Check each option

• A. Interference is the superposition of two or more waves, giving reinforcement or cancellation. It is not the spreading of a wave through a gap.
• B. Dispersion is the splitting of white light into its component colours, for example by a prism.
• C. Refraction is the change in direction of a wave when its speed changes as it enters a different medium. It does not involve a gap.
• D. Diffraction is the bending and spreading of waves as they pass through a gap or around an edge. A narrow gap makes plane waves emerge as circular waves, so this matches the question.

Step 4: Answer The correct option is D. diffraction.

Exam tip: Associate gap or edge → diffraction, different medium → refraction, two waves meeting → interference, colours splitting → dispersion. In a ripple tank, the narrower the gap, the more circular the emerging waves.