Let focal length of converging lens f₁ = + f and focal length of diverging lens f₂ = -f. If focal length of combination be feq, then 1/feq = 1/f1 + 1/f2 = 1/(+f) + 1/(-f) = 0
feq = 00 or Peq = 0.
Class 12 Physics
Chapter 9 Ray Optics and Optical Instruments Session 2024-2025.
A converging lens is kept coaxially in contact with diverging lens – both the lenses being of equal focal lengths. What is the focal length of the combination?
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For a converging lens (f1 >0) and a diverging lens (f2 <0) with equal focal lengths (∣f1 ∣= ∣f2∣):
1/f = 1/f1 + 1/f2 = 1/f – 1/f = 0
Thus, the combination has infinite focal length and behaves like a plane glass.
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