For a combination of two convex lenses of focal lengths $f_1$ and $f_2$ to act as a glass slab, the distance of separation between them is

If a ray of light passes through an equilateral prism such that the angle of incidence and the angle of emergence are both equal to $70 \%$ of the angle of the prism, then the angle of minimum deviation is

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