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

Refractive index of a glass convex lens is 1.5. The radius of curvature of each of the two surfaces of the lens is $$20 \mathrm{~cm}$$. The ratio of the power of the lens when immersed in a liquid of refractive index 1.25 to that when placed in air is

When a monochromatic ray of light is passed through an equilateral glass prism, it is found that the refracted ray in glass is parallel to the base of the prism. If '$$i$$' and '$$e$$' denote the angles of incidence and emergence respectively, then

A combination of two thin lenses in contact have power $$+10 \mathrm{D}$$. The power reduces to $$+6 \mathrm{D}$$ when the lenses are $$0.25 \mathrm{~m}$$ apart. The power of individual lens is