An achromatic convergent doublet of two lenses in contact has a power of +5 D. The power of converging lens is +6 D. The ratio of the dispersive power of the convergent and divergent lenses is

The property of light used in optical fibre cables is

Two plane mirrors A and B are aligned parallel to each other as shown in the figure.

A ray of light is incident at an angle 30$$^\circ$$ at a point just inside one end of A. The plane of incidence coincides with the plane of the figure. The maximum number of times, the ray undergoes reflection (excluding the first one) before it emerges out is

A thin equi-convex lens is made of glass of refractive index 1.5 and its focal length is 0.2 m. If it acts as a concave lens of focal length 0.5 cm when dipped in a liquid, the refractive index of the liquid is