In the H–He⁺ mixture experiment, the wavelength of light emitted in the visible region by He⁺ ions after collisions with H atoms is

Visible region → Balmer-like series for He⁺: n=4 to n=2. ΔE = 13.6×4×(1/4 − 1/16) = 54.4 × (3/16) = 10.2 eV. λ = hc/E = (6.626×10⁻³⁴ × 3×10⁸)/(10.2×1.6×10⁻¹⁹) ≈ 1.22×10⁻⁷/1.632×10⁻¹⁸... Recalculate: ΔE(4→2 He⁺) = 13.6×4(1/4−1/16) = 10.2 eV. λ = 1240 eV·nm/10.2 eV ≈ 121.6 nm — UV! Actually He⁺ Balmer: n=4→n=2: ΔE = 13.6 × 4 × (1/4 − 1/16) = 10.2 eV, λ ≈ 122 nm. For visible: recalculate n=4→n=3: ΔE = 54.4(1/9−1/16) = 54.4 × 7/144 = 2.64 eV, λ = 469 nm ≈ 4.7×10⁻⁷ m ≈ 4.8×10⁻⁷ m.