You are given three lenses L 1 , L 2 and L 3 each of focal length 20 cm. An object is kept at 40 cm in front of L 1 , as shown. The final real image is formed at the focus ‘I’ of L 3 . Find the separations between L 1 , L 2 and L 3 . - Wired Faculty

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You are given three lenses L₁, L₂ and L₃each of focal length 20 cm. An object is kept at 40 cm in front of L₁, as shown. The final real image is formed at the focus ‘I’ of L₃. Find the separations between L₁, L₂ and L₃.

Solution

Given,
f₁ = f₂ = f₃ = 20 cm

In case of lens L₁,

Object distance, u₁ = -40 cm

Using Lens formula,
$space space space space space 1 over straight v subscript 1 space minus space 1 over straight u subscript 1 space equals space 1 over straight f subscript 1 rightwards double arrow space 1 over straight v subscript 1 space equals space 1 over 20 space minus space minus 1 over 40 rightwards double arrow space space straight v subscript 1 space equals space 40 space cm$
For lens L₃,
$straight f subscript 3 space equals space 20 space cm. space straight v subscript 3 space equals space 20 space cm Object space distance comma space straight u subscript 3 space equals space ? Using space lens space formula comma space we space have 1 over straight v subscript 3 minus 1 over straight u subscript 3 space equals space 1 over straight f subscript 3 space rightwards double arrow space 1 over 20 minus 1 over straight u subscript 3 space equals space 1 over 20 rightwards double arrow space space 1 over straight u subscript 3 space equals 0 space rightwards double arrow space space space space straight u subscript 3 space equals space infinity$

So, lens L₂ should produce the image at infinity. Thus, objective should be at focus. The image formed by lens L₁ is at 40 cm on the right side of lens L₁ which lies at 20 cm left of lens L₂ i.e., focus of lens L₂.

Hence, the distance between L₁and L₂ = 40 + 20 = 60 cm.

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Wave Optics

You are given three lenses L₁, L₂ and L₃each of focal length 20 cm. An object is kept at 40 cm in front of L₁, as shown. The final real image is formed at the focus ‘I’ of L₃. Find the separations between L₁, L₂ and L₃.

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Wave Optics

You are given three lenses L1, L2 and L3 each of focal length 20 cm. An object is kept at 40 cm in front of L1, as shown. The final real image is formed at the focus ‘I’ of L3. Find the separations between L1, L2 and L3.

Some More Questions From Wave Optics Chapter

Mock Test Series

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Entrance Exams Preparation

You are given three lenses L₁, L₂ and L₃each of focal length 20 cm. An object is kept at 40 cm in front of L₁, as shown. The final real image is formed at the focus ‘I’ of L₃. Find the separations between L₁, L₂ and L₃.