Q. Two glass prisms $P_1$ and $P_2$ are to be combined together to produce dispersion without deviation. The angles of the prisms $P_1$ and $P_2$ are selected as $4^{\circ}$ and $3^{\circ}$ respectively. If the refractive index of prism $P_1$ is $1.54$, then that of $P_2$ will be
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