A thin prism $P_1$ with angle $6°$ and made from glass of refractive index $1.54$ is combined with another thin prism $P_2$ of refractive index $1.72$ to produce dispersion without deviation. The angle of prism $P_2$ will be

A ray of light is incident on an equilateral glass prism placed on a horizontal table. For minimum deviation which of the following is true?

A diverging beam of light from a point source $S$ having divergence angle $\alpha$, falls symmetrically on a glass slab as shown in figure. The angles of incidence of the two extreme rays are equal. If the thickness of the glass slab is $t$ and the refractive index $n$, then the divergence angle of the emergent beam is