Question

60. If the focal length of the lens is 4 cm and
object is placed at 6 cm from optical centre
the image will be formed at:
(a) 2.4 cm from O (b) 4.8 cm from O
(c) 24 cm from O (d) 48 cm from O​

Answer 1

Answer:

V= 2.4 cm

Explanation:

F= 4cm

U= 6cm

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

apply lens formula

you will get your answer. For reference i have provided you with the solution

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Answer 2

Given info : If the focal length of the lens is 4 cm and the object is placed at 6 cm from the optical centre.

To find : The image will be formed at, is ..

solution : here it is not mentioned that which lens are being used. so we should answer this question with two cases.

case 1 : when lens is concave lens.

focal length, f = - 4 cm

object distance, u = -6 cm

using formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

⇒$\frac{1}{v}-\frac{1}{-6}=\frac{1}{-4}$

⇒$\frac{1}{v}=\frac{1}{-6}+\frac{1}{-4}$

⇒v = -2.4 cm

Therefore the image will be formed at 2.4 cm from O. option (1) is correct.

case 2 : when lens is convex lens.

f = +4 cm

u = - 6cm

using formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

⇒$\frac{1}{v}-\frac{1}{-6}=\frac{1}{4}$

⇒v = +12 cm

Therefore the image will be formed at 12 cm from O. no option is given..

Therefore the correct answer is option (1).

[ note : The required lens is concave lens ]