• summary_type: application
• word_count: 160

The refractive index of a prism is determined using minimum deviation: n = sin((A + D_m)/2)/sin(A/2). At minimum deviation: i = e (angle of incidence equals angle of emergence), r_1 = r_2 = A/2 (symmetric passage), and the internal ray is parallel to the prism base. Experiment: use a spectrometer to measure deviation angle for various incidence angles. Plot deviation vs incidence — the curve has a minimum (D_m). For a 60 degree prism with n = 1.5: D_m = 37.2 degrees; with n = sqrt(2): D_m = 30 degrees. JEE often provides A and D_m and asks for n, or gives n and A to find D_m. The formula works because at minimum deviation, the geometry simplifies: applying Snell's law at both surfaces with the symmetric condition directly yields the formula. For thin prisms (small A): D ≈ (n-1)*A, independent of angle of incidence.