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 n_d = 1.5159 and a ν-number = 70.0, and the flint glass have a refractive index n_d = 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

(4.1)

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.