Question

# A thin bi-convex lens of focal length 20 cm is made of glass of refractive index 1.5. When it is dipped in a liquid of refractive index 98, it acts as a:

A
convex lens of focal length 150 cm
No worries! Weâ€˜ve got your back. Try BYJUâ€˜S free classes today!
B
concave lens of focal length 30 cm
No worries! Weâ€˜ve got your back. Try BYJUâ€˜S free classes today!
C
convex lens of focal length 30 cm
Right on! Give the BNAT exam to get a 100% scholarship for BYJUS courses
D
concave lens of focal length 15 cm
No worries! Weâ€˜ve got your back. Try BYJUâ€˜S free classes today!
Open in App
Solution

## The correct option is D convex lens of focal length 30 cmThe focal length of a lens is given by the formula, 1f=n2−n1n1(1R1−1R2)Hence, in air, 1f=1.5−11(1R+1R)=1R. Thus f=R=20 cm.In the medium, 1f′=1.5−9898(1R+1R)=23RHence f=1.5R=30 cmSince the focal length is coming out to be positive. It behaves like a convex lens of focal length 30 cm.

Suggest Corrections
0
Related Videos
Other Cases of Diffraction
PHYSICS
Watch in App