The focal lengths of the objective and the eyepiece of a compound microscope are 2 cm and 3 cm respectively and the distance between them is 15 cm . The final image formed by the eyepiece is at infinity. The distances of the object and the image produced by the object from the objective lens are respectively.

Two convex lenses of focal lengths 20 cm and 30 cm are placed in contact with each other co-axially. The focal length of the combination is

A convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black, then

Refractive index of a medium is $\mu$. If the angle of incidence is twice that of the angle of refracation, then the angle of incidence is