A beam of polychromatic light passes through a thin prism of prism angle 6°. The refractive index of the material of the prism varies with wavelength (λ) as n(λ) = αλ + β/λ², where α = 3 μm⁻¹ and β = 0.096 μm². If λ_min is the wavelength at which the angle of minimum deviation D_m is smallest, then the correct value of D_m at λ_min is