F1=f11+f21−f1f2d
∴∞1=201+−51−20(−5)d
⇒0=201−51+100d
∴100d=51−201=10020−5
∴d=15cmFrom figure it is clear that separation between lenses, d=20−5=15cm 
A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with a distance d between them. If a…
NEET UG 2021 — Physics Optics
2021mcqhard
A convex lens A of focal length 20cm and a concave lens B of focal length 5cm are kept along the same axis with a distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be :
Official previous-year question
Held on 30 Apr 2021 · Verified 9 Jul 2026.
Options
- A
50
- B
15
- C
25
- D
30
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