Single Slit Ensures Coherent Wavefront for Interference

Steps:

• Light from the source passes through the single slit, which diffracts it into a cylindrical wavefront.
• This wavefront illuminates both double slits uniformly with the same phase.
• The double slits then act as coherent sources, producing stable interference fringes.
• Without coherence, phase differences fluctuate, washing out the pattern.

Why C is correct:

• Coherence means constant phase difference between waves; the single slit creates a single wavefront source, ensuring waves from the double slits remain in phase per Huygens' principle.

Why the others are wrong:

• A: Narrowing occurs via diffraction, but the primary role is coherence, not beam width.
• B: Equal amplitude results from symmetric illumination, not the slit's direct function.
• D: Monochromaticity comes from the light source or filters, not the slit.

Final answer: C