What is the position and nature of image formed by lens combination shown in figure? (f1, f2 are focal lengths) (a) 20/3 cm from point B at right, real (b) 70 cm from point B at right; real (c) 40 cm from point B at right; real (d) 70 cm from point B at left; virtual

Answer: (b). Lens A: u = -20, f = +5: 1/v = 1/5 - 1/20 = 3/20 ⇒ v = 20/3 cm. This is 20/3 - 2 = 14/3 cm right of B: virtual object, u = +14/3. Lens B: 1/v = -1/5 + 3/14 = 1/70 ⇒ v = +70 cm So the answer is (b) 70 cm right of B, real.

What is the position and nature of image formed by lens combination… | JEE Main 2019 (12 Jan, Shift 1)

Q55JEE Main 2019 · 12 JAN · SHIFT 1Medium

Combination of Lenses

What is the position and nature of image formed by lens combination shown in figure? (f1, f2 are focal lengths)

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Answer: (b)

Lens A: $u = -20$ , $f = +5$ : $\frac{1}{v} = \frac{1}{5} - \frac{1}{20} = \frac{3}{20} \Rightarrow v = \frac{20}{3}\text{ cm}$ .
This is $\frac{20}{3} - 2 = \frac{14}{3}\text{ cm}$ right of B: virtual object, $u = +\frac{14}{3}$ .
Lens B: $\frac{1}{v} = -\frac{1}{5} + \frac{3}{14} = \frac{1}{70} \Rightarrow v = +70\text{ cm}$
So the answer is (b) 70 cm right of B, real.

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