Diffraction grating maxima follow the grating equation d sinθ = mλ

Steps:

• Monochromatic light has fixed wavelength λ, producing discrete intensity maxima.
• Grating equation d sinθ = mλ gives angles θ for integer orders m = 0, ±1, ±2, ...
• Normal incidence yields symmetric maxima: central (m=0) straight ahead, pairs at ±θ for |m| ≥ 1.
• Maximum intensity occurs only at these predicted directions, not elsewhere.

Why C is correct:

• Shows central beam (m=0) plus symmetric first- and second-order pairs on both sides, matching d sinθ = mλ for multiple orders.

Why the others are wrong:

• A: Only central maximum, omits diffracted orders required by grating equation.
• B: Asymmetric single-side beams, violates symmetry for normal incidence.
• D: Unequal spacing of beams, contradicts equal-angle pairs for integer m.

Final answer: C