Ray Optics Questions

Two similar thin equi-convex lenses, of focal length $f$ each, are kept coaxially in contact with each other such that the focal length of the combination is $F_1$. When the space between the two lenses is filled with glycerin (which has the same refractive index ($\mu =1.5$) as that of glass) then the equivalent focal length is $F_2$. The ratio $F_1:F_2$ will be:

Pick the wrong answer in the context with rainbow.

In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?

A biconvex lens has power $P$. It is cut into two symmetrical halves by a plane containing the principal axis. The power of one part will be:

A double convex lens has a focal length of $25\text{cm}$. The radius of curvature of one of the surfaces is double of the other. What would be the radii if the refractive index of the material of the lens is $1.5$?