A convex lens and a concave lens of focal lengths 15 cm and 20 cm respectively are placed in contact with each other. When an object is placed at 60 cm from the combination, image is formed at

Question

A convex lens and a concave lens of focal lengths 15 cm and 20 cm respectively are placed in contact with each other. When an object is placed at 60 cm from the combination, image is formed at (A) 60 cm (B) 30 cm (C) 120 cm (D) Infinity

Tardigrade Admin · Accepted Answer

<img class="img-fluid question-image" alt="image" src="https://cdn.tardigrade.in/img/question/physics/2972c08457f11c8c4a3d529fc21e4b0e-.png" /> Combined focal length (1/f)=(1/15)+(1/(-20))=(4-3/60) f=60 cm ( a convex lens) (1/v)-(1/u)=(1/f) (1/v)+(1/60)=(1/60) v=∞ (image at infinity), because object is at focus of convex lens.

Q. A convex lens and a concave lens of focal lengths $15 c m$ and $20 c m$ respectively are placed in contact with each other. When an object is placed at $60 c m$ from the combination, image is formed at

60 cm

30 cm

120 cm

Infinity

Combined focal length
$\frac{1}{f} = \frac{1}{15} + \frac{1}{( - 20 )} = \frac{4 - 3}{60}$
$f = 60 c m$ ( a convex lens)
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{v} + \frac{1}{60} = \frac{1}{60}$
$v = \infty$ (image at infinity), because object is at focus of convex lens.

Q. A convex lens and a concave lens of focal lengths 15cm and 20cm respectively are placed in contact with each other. When an object is placed at 60cm from the combination, image is formed at