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

The refractive index is found using minimum deviation: n = sin((A + D_m)/2)/sin(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 = (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.