    Question

# You are given two thin identical plano-convex lenses. When you place an object 20 cm to the left of a single plano-convex lens, the image appears 40 cm to the right of the lens. You then arrange the two plano-convex lenses back to back to form a double convex lens. If the object is at 20 cm to the left of this new lens, what is the approximate location of the image ? Given f=2R for plano-convex lens A
6.7 cm to the right of the lens
No worries! We‘ve got your back. Try BYJU‘S free classes today!
B
10 cm to the right of the lens
Right on! Give the BNAT exam to get a 100% scholarship for BYJUS courses
C
20 cm to the right of the lens
No worries! We‘ve got your back. Try BYJU‘S free classes today!
D
80 cm to the right of the lens
No worries! We‘ve got your back. Try BYJU‘S free classes today!
Open in App
Solution

## The correct option is B 10 cm to the right of the lensFor plano convex lens - u=−20 cm, v=+40 cm, f=2R 1v−1u=1f (lens formula) 140+120=12R R=203 For the convex lens u=−20 cm, f=R=203 Using lens formula, 1v−1u=1f 1v+120=320 1v=320−120 1v=3−120=220 v=10 cm Why this question? Concept: 1f=(μ−1)(1R1−1R2) is the formula for finding focal length for all kinds of lenses placed in air.  Suggest Corrections  1      Similar questions  Explore more