pl. answer...

Pls post each question seperately.

Pls note that , "Dioptre" - the unit of power of a lens is denoted by capital 'D'.

(a) Positive power indicates that the lens is convex.Hence, in lenses having powers of + 2 D and +4 D are convex  lenses.

 focal length ( in metres) = 1 / power of the lens.

so, focal length of 1st lens , f₁ = 1/ 2 m

and, focal length of 2nd lens , f ₂ = 1 / 4 m.