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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 - CBSE Class 10 - Science

ConceptRefraction of Light Image Formation by Lenses - Convex Lens

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) = P1 + P2
Net power (P) = 10 D-5 D = 5 D

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

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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.
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