Diffraction grating equation determines line density

Steps:

• Apply the grating equation: d sin θ = m λ, so λ/d = sin θ / m.
• For the second-order maximum, m = 2, yielding λ/d = sin θ / 2.
• The graph of λ/d versus sin θ is linear with slope 1/2, confirming the order but independent of d.
• No numerical values for λ or θ (or scales from the graph) are provided to compute d = 2 λ / sin θ.

Not enough information.

Why A is correct:

• Cannot determine; lacks data to calculate 1/d = 400 mm^{-1}.

Why the others are wrong:

• B: Insufficient data for 625 mm^{-1}.
• C: Insufficient data for 800 mm^{-1}.
• D: Insufficient data for 1250 mm^{-1}.

Final answer: Not enough information.