Which one among the following is the correct focal length of a combination of lenses of power 2.5 D and -2.0 D ?

In the dispersion of white light by a common glass prism, which one among the following is correct?

A lens has a power of +2.0 Dioptre. Which one of the following statements about the lens is true ?

Which one among the following is correct for a person suffering from myopia ?

Which one of the following statements about the aperture of a convex lens is correct?

Which one of the following statement is not correct?