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# A thin prism of angle $6^{\circ}$ made up of glass of refractive index 1.5 is combined with another prism made up of glass of refractive index 1.75 to produce dispersion without deviation. The angle of second prism is :

$\begin {array} {1 1} (1)\;7^{\circ} & \quad (2)9^{\circ} \\ (3)\;4^{\circ} & \quad (4)\;5^{\circ} \end {array}$

Ans : (3)
$4^{\circ}$