Spectrometer

The spectrometer is an optical instrument used to study the spectra produced by different light sources and to measure the refractive indices of materials. It consists of three main parts: (i) the collimator, an arrangement producing a parallel beam of light -- a long cylindrical tube with a convex lens at its inner end and an adjustable-width vertical slit at its outer end, with the slit-to-lens distance set so the slit sits exactly at the lens's focus; (ii) the prism table, used for mounting the prism, grating, etc., consisting of two circular metal discs on three levelling screws, rotatable about a vertical axis and readable via two verniers $V_1,V_2$, adjustable in height; and (iii) the telescope, an astronomical-type instrument with cross-wires in its eyepiece and an objective lens, whose separation is adjustable so a clear image forms at the cross-wires for an incoming parallel beam; the telescope is mounted on a rotating arm with a graduated circular scale (in half-degree divisions), with both telescope and prism table fitted with radial screws (for coarse fixing) and tangential screws (for fine adjustment). Four preliminary adjustments precede any measurement: (a) adjusting the eyepiece by turning the telescope towards an illuminated surface until the cross-wires are seen sharply; (b) adjusting the telescope, by turning it towards a distant object, to …