A transmission grating has 5000 lines per cm. Light of wavelength 589 nm falls normally on it. (a) Find the grating element. (b) At what angle does the second-order maximum appear? (c) What is the highest order visible? Solution: (a) Grating element (a + b) = 1 cm / 5000 = 2.0 × 10⁻⁶ m. (b) (a + b) sin θ = nλ → sin θ = 2 × 589×10⁻⁹ / 2.0×10⁻⁶ = 0.589 → θ ≈ 36.0°. (c) Maximum order: sin θ ≤ 1 → n_max = (a + b)/λ = 2.0×10⁻⁶ / 589×10⁻⁹ ≈ 3.39, so orders n = 0, ±1, ±2, ±3 are visible; the 4th order cannot form.