#### Question

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:

(a) +10 cm

(b) +20 cm

(c) −10 cm

(d) −20 cm

#### Solution

+20 cm

We know that:

Power (P) = `1/"focal length"`

Therefore

Power of convex lens `p_1=1/(10cm)=1/(0.1m)=+10D`

Power of convave lens `P_2=1/(-20cm)=-1/(0.2m)=-5D`

We know that power of lens adds up when lenses comes in contact. Therefore, we have:

Net power (P) = P_{1} + P_{2}

Net power (P) = 10 D-5 D = 5 D

Net focal length = `1/"power"=1/5=0.2m=20cm`

Is there an error in this question or solution?

Solution A Convex Lens of Focal Length 10 Cm is Placed in Contact with a Concave Lens of Focal Length 20 Cm. the Focal Length of this Combination of Lenses Will Be: (A) +10 Cm (B) +20 Cm (C) −10 Cm (D) −20 Cm Concept: Refraction of Light - Image Formation by Lenses - Convex Lens.