Question

Two sources are called coherent if they produce waves

• A. of equal wavelength
• B. of equal velocity
• C. having a constant phase difference
• D. having same shape of wavefront

Hint:

Coherence = Same frequency + Constant phase difference → Required for sustained interference patterns.

Answer 1

The Correct Option is C

Concept: Two sources are said to be coherent if they emit waves that maintain a constant phase difference over time. This property is essential for producing a stable interference pattern.

Step 1: Definition of coherent sources.
Coherent sources must:
• Have the same frequency (or wavelength)
• Maintain a constant phase difference

Step 2: Analyze the given options.

• Equal wavelength → not sufficient alone
• Equal velocity → not relevant condition
• Constant phase difference → correct condition
• Same wavefront shape → not defining criteria

Step 3: Final conclusion.
Thus, the defining condition for coherent sources is: \[ having a constant phase difference \]