Question

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

(a) 20 cm in front of the lens
(b) 8 cm in front of the lens
(c) 15 cm in front of the lens
(d) 60 cm in front of the lens​

Answer 1

Answer:

c is correct answer

Explanation:

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Answer 2

Given: Sakshi 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 produce the sharpest image.

To find: The distance of the object from the lens.

Solution:

• According to the lens maker's formula, refraction of light through a thin lens is given by,

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

• Here, v is the image distance, u is the object distance from the lens and f is the focal length of the lens.
• Since the image is to be formed on the screen, the distance of the screen from the lens will be the image distance.
• On replacing the terms with the values given in the question, the obtained equation is,

        $\frac{1}{20 cm} - \frac{1}{u} = \frac{1}{15 cm}$

        $\frac{1}{u} = - \frac{1}{60}$

• The object distance for a convex lens is always negative because the object is placed to the left of the lens.

• So, the minus sign indicates that the object is kept in front of the lens.

Therefore, the distance of the object from the lens is 60 cm in front of the lens.