Question

Q29 Rakhi conducts an experiment to produce an image of an object on a screen which is placed at 20 cm from the lens. She uses a convex lens of focal length 15 cm for the experiment. Where should she place the object in order to produce the sharpest image?
(a) 60 сm in front of the lens
(b) 20 cm in front of the lens
(c) 15 cm in front of the lens
(c) 8 cm in front of the lens ​

Answer 1

Answer:

A

Explanation:

lens formula ⇒ 1/f = 1/v - 1/u

f = +15cm  

v = +20cm  

u = ?

putting values,

1/15 = 1/20 - 1/u

1/15 - 1/20 = -1/u

1/60 = -1/u

u = -60cm

Answer 2

Given:

Rakhi conducts an experiment to produce an image of an object on a screen which is placed at 20 cm from the lens. She uses a convex lens of focal length 15 cm.

To find:

Object distance from lens ?

Calculation:

Applying LENS FORMULA:

$\sf \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$\sf \implies\dfrac{1}{15} = \dfrac{1}{20} - \dfrac{1}{u}$

$\sf \implies\dfrac{1}{u} = \dfrac{1}{20} - \dfrac{1}{15}$

$\sf \implies\dfrac{1}{u} = \dfrac{3 - 4}{60}$

$\sf \implies\dfrac{1}{u} = \dfrac{ - 1}{60}$

$\sf \implies u = - 60 \: cm$

So, image distance is 60 cm in front of lens.