Question

two lenes have power (i) +2D (ii) -4D. what is the nature and power of this combination of lens.

Answer 1

Heya,

Nature of the lens means that what type of lens it is, so ;

(i) Lens that have positive power means that it is a convex lens. So, first lens with power +2D is a convex lens.

(ii) Lens that have negative power means that it is concave lens. So, second lens with power -4D is a concave lens.

To calculate the power of combination we have to just add the power of lens given.
First lens power (P1) = +2D
Second lens power(P2) = -4D

So, total = P1 + P2
= (+2)+(-4)
= -2D

So, power of combination is -2D. So, nature of combination will be of a concave lens.

Hope this helps....:)