• $summary_{type}$: application
• $word_{count}$: 160

The refractive index of a prism is determined using minimum deviation: n = sin$\frac{(A + D_m}{2}$)/sin$\frac{A}{2}$. 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.