• summary_type: concept
• word_count: 170

A prism of angle A causes deviation δ = i_{1} + i_{2} - A, with the constraint r_{1} + r_{2} = A inside the prism. Minimum deviation occurs when i_{1} = i_{2} (symmetric passage), giving the important formula n = sin((A + δ_m)/2)/sin(A/2). For thin prisms (small A), the deviation simplifies to δ = (n-1)A, which is independent of the angle of incidence. The deviation versus incidence (δ-i) curve is U-shaped with a single minimum at δ_m. For each δ > δ_m, there are two possible angles of incidence. The condition for a ray to emerge from the second surface is A < 2θ_c (where θ_c is the critical angle). If A exceeds 2θ_c, total internal reflection occurs at the second face. JEE frequently tests minimum deviation calculations, typically requiring students to find the refractive index or the angle of incidence at minimum deviation.