Len's maker's formula
f1=(μ−1)(R11−R21)
f11=(μ−1)201⇒(1.5−1)201=401
f11=401
For lens f11+f21=401+401=201
For concave f31=(1.7−1)(−R1−R1)=R−0.7×2
feq1=201+(R−0.7×2)
=201−201.4
feq=−0.420=−50cm
NEET UG 2015 — Physics Optics
Two identical thin Plano-convex glass lenses (refractive index 1.5 ) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the center. The intervening space is filled with oil of refractive index 1.7 The focal length of the combination is
Held on 30 Apr 2015 · Verified 9 Jul 2026.
−20cm
−25cm
−50cm
50cm
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