Prism Formula Minimum Deviation — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Prism Formula (Minimum Deviation) Applicable for refraction through a triangular prism when the angle of incidence allows the light ray to pass symmetrically through the prism. Start with general prism relations: A = r1 + r2 and δ = i + e - A. Apply condition for minimum deviation: i = e and r1 = r2 = r. Substitute conditions into relations: A = 2r => r = A/2 and δm = 2i - A => i = (A + δm)/2. Apply Snell's Law at the first interface: μ = sin(i) / sin(r). Substitute expressions for i and r into Snell's Law. The prism must be in the position of minimum deviation (angle of incidence equals angle of emergence). Light must be monochromatic as refractive index varies with wavelength. The surrounding medium is assumed to be air unless is defined as a relative refractive index. Students often forget that this formula is only valid specifically at minimum deviation, not for general refraction. Confusing the prism angle A with the critical angle. Assuming the ray inside the prism is always parallel to the base (only true for isosceles/equilateral prisms at minimum deviation).