• summary_type: concept
• word_count: 160

Dispersion is the separation of white light into constituent wavelengths due to the wavelength dependence of refractive index (Cauchy's equation: n ≈ A + B/λ^{2}). Angular dispersion through a thin prism is (n_V - n_R)A, and dispersive power ω = (n_V - n_R)/(n_Y - 1). Two important prism combinations exist: (1) Achromatic combination (deviation without dispersion): ω_{1}δ_{1} + ω_{2}δ_{2} = 0, where the prisms have different dispersive powers. (2) Direct vision combination (dispersion without deviation): (n_{1}-1)$A_{1}$ + (n_{2}-1)$A_{2}$ = 0 for mean wavelength. In both cases, the prisms are oriented with their angles opposing. These combinations are frequently tested in JEE as 2-mark problems requiring setup of simultaneous equations.