The prism has refracting angle '$$\mathrm{A}$$'. The second refracting surface of the prism is silvered. Light ray falling on first refracting surface with angle of incidence '$$2 \mathrm{~A}$$', reaches the second surface and returns back through the same path due to reflection at the silvered surface. The refractive index of the material of the prism is

To get three images of a single object, the angle between the two plane mirrors should be

Two lenses of power $$-15 \mathrm{D}$$ and $$+5 \mathrm{D}$$ are in contact with each other. The focal length of the combination is

Which of the following is NOT involved in the formation of secondary rainbow?