The biconvex lens is a combination of two thin plano-convex lenses in contact.
For the first plano-convex lens: μ1=1.5, R1=15 cm, R2=∞.
Using lens maker's formula:
f11=(μ1−1)(R11−R21)
=(1.5−1)(151−∞1)
=0.5×151=301 cm−1.
For the second plano-convex lens:
μ2=1.2, R1=∞, R2=−12 cm.
Using lens maker's formula:
f21=(μ2−1)(R11−R21)
=(1.2−1)(∞1−−121)
=0.2×121=601 cm−1.
The equivalent focal length F of the combination is given by:
F1=f11+f21=301+601
=602+1=603=201 cm−1.
Thus, F=20 cm.
Given object distance u=−30 cm. Using the lens formula v1−u1=F1:
v1−−301=201
⇒v1=201−301=603−2=601.
So, v=60 cm.
Magnification m=uv=−3060=−2.
