Coherence for Interference in Diffraction Gratings

Steps:

• Diffraction gratings produce interference patterns based on constructive interference of light waves from multiple slits.
• For clear maxima and minima, incident waves must maintain constant phase difference over time.
• Incoherent light causes patterns to wash out, preventing wavelength measurement.
• Thus, coherence ensures stable fringes for accurate angle and wavelength calculation via d sinθ = mλ.

Why C is correct:

• Coherence means waves have fixed phase relationship, essential for the grating equation d sinθ = mλ to yield precise interference maxima.

Why the others are wrong:

• A: Flat screens suffice; curvature is unnecessary for standard setups.
• B: Single maximum with grating spacing allows λ determination; multiple not required.
• D: Any order maximum works; third order is arbitrary and often weak.

Final answer: C