An object is $$8 \mathrm{~cm}$$ high. It is desired to form a real image $$4 \mathrm{~cm}$$ high at $$60 \mathrm{~cm}$$ from the mirror. The type of mirror needed with the focal length is

When an object is placed $$40 \mathrm{~cm}$$ from a diverging lens, its virtual image is formed $$20 \mathrm{~cm}$$ from the lens. The focal length and power of lens are

A person has a minimum distance of distinct vision as $$50 \mathrm{~cm}$$. The power of lenses required to read a book at a distance of $$25 \mathrm{~cm}$$ is