The focal length of combination of lenses formed with lenses having power of +2.50 D and $-3.75$ D will be

Two thin lenses have a combined power of +9D. When they are separated by a distance of 20 cm , their equivalent power becomes $+\frac{27}{5} \mathrm{D}$. The power of both the lenses in dioptre are respectively

A plane mirror produces a magnification of

The telescopes, for a given wavelength, the objectives with large aperture are used for