Q

Two lenses, one converging with focal length 12 cm and one diverging with focal length −18 cm, are placed in contact. What is the combined focal length of this lens system?

RRB Group D 18 Aug, 2026 (12:45 PM - 2:15 PM) 2026
✓ Answer: A 36 cm
For thin lenses in contact, 1/F = 1/f₁ + 1/f₂ = 1/12 − 1/18 = 1/36, so the combined focal length is 36 cm.
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