4.1 Dispersing Prism
4.1.1 Refracting direct-vision prism
A direct-vision prism disperses the input light with no significant angular deviation of the output. It is often called a direct-view spectroscope. The simplest type of direct-vision prism is shown in Fig. 4.1, where the displaced and dispersed output is coaxial with the input. This compound prism usually consists of crown- and flint-glass prisms cemented at the interface surface. Let the crown glass have a refractive index nd = 1.5159 and a ν-number = 70.0, and the flint glass have a refractive index nd = 1.9626 and a ν-number = 19.7. If we choose the slope angle α of the interface, then the vertex angle β of the first prism can be calculated from
where n1 is the design refractive index of the crown glass, and n2 is the design refractive index of the flint glass.1 To produce a dispersed output that is collinear with the input, two of the above prisms are combined to form a double Amici prism (Fig. 4.2). Using the same crown and flint glasses, if we choose α = 45.0 deg, then β = 98.123 deg.