For the thin convex lens, the radii of curvature are at $$15 \mathrm{~cm}$$ and $$30 \mathrm{~cm}$$ respectively. The focal length the lens is $$20 \mathrm{~cm}$$. The refractive index of the material is :

An effective power of a combination of 5 identical convex lenses which are kept in contact along the principal axis is $$25 \mathrm{D}$$. Focal length of each of the convex lens is:

In an experiment to measure focal length ($$f$$) of convex lens, the least counts of the measuring scales for the position of object (u) and for the position of image (v) are $$\Delta u$$ and $$\Delta v$$, respectively. The error in the measurement of the focal length of the convex lens will be:

The refractive index of a prism with apex angle $$A$$ is $$\cot A / 2$$. The angle of minimum deviation is :