• Tags: prism, minimum-deviation, refractive-index
• Difficulty: Moderate

The refractive index is found using minimum deviation: n = sin$\frac{(A + D_m}{2}$)/sin$\frac{A}{2}$. Experiment: set up the spectrometer with the prism on the prism table. Send a collimated beam through the prism and observe the deviated beam through the telescope. Rotate the prism slowly — the deviation first decreases, reaches a minimum ($D_m$), then increases. At minimum deviation, the ray passes symmetrically through the prism: angle of incidence = angle of emergence, and the ray inside is parallel to the base. Measure the angle of prism A using the reflected beam method: direct the beam at both faces and measure the angle between reflections; A = $\frac{angle}{2}$ if measured as the supplement, or directly from the geometry. Plot deviation vs incidence angle to identify $D_m$ graphically. For a standard glass prism (n = 1.5) with A = 60 degrees: $D_m$ = 37.2 degrees.