Intensity and Amplitude in Coherent Waves

Steps:

• Coherence requires waves to have the same frequency for stable interference.
• Electromagnetic waves in vacuum travel at the speed of light, so same speed.
• Constant phase difference is essential for coherence at the meeting point.
• Intensity I is proportional to amplitude A squared (I ∝ A²), so different intensities imply different amplitudes.

Why B is correct:

• Different intensities mean different amplitudes, as I = (1/2) c ε₀ E₀² where E₀ relates to amplitude.

Why the others are wrong:

• A: Coherence demands constant phase difference for predictable interference.
• C: Coherence requires identical frequencies.
• D: All electromagnetic waves in vacuum propagate at speed c.

Final answer: B