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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Geometrical Optics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Geometrical Optics

A converging lens with a focal length of 15 cm and a converging...

A converging lens with a focal length of 15 cm15 \text{ cm} and a converging mirror with a focal length of 10 cm10 \text{ cm} are positioned 50 cm50 \text{ cm} apart, sharing a common principal axis. A point source is situated between the lens and the mirror, at a distance of 40 cm40 \text{ cm} from the lens. Determine the locations of the two images formed.

Options

  1. A24 cm24 \text{ cm} from the lens towards the mirror, and 15 cm15 \text{ cm} from the lens away from the mirror
  2. B15 cm15 \text{ cm} from the lens towards the mirror, and 24 cm24 \text{ cm} from the lens away from the mirror
  3. C15 cm15 \text{ cm} and 24 cm24 \text{ cm} from the lens, both on the side away from the mirror
  4. D10 cm10 \text{ cm} and 30 cm30 \text{ cm} from the lens, both on the side away from the mirror

Correct answer

C. 15 cm15 \text{ cm} and 24 cm24 \text{ cm} from the lens, both on the side away from the mirror

Step-by-step solution

The light from the point source can form an image in two ways: by directly refracting through the lens, and by first reflecting from the mirror and then refracting through the lens.

For the first image (formed directly by the lens):
The source is at a distance of 40 cm40 \text{ cm} from the lens.
Using the lens formula 1v1−1u=1f\dfrac{1}{v_1} - \dfrac{1}{u} = \dfrac{1}{f}:
1v1−1−40=115\dfrac{1}{v_1} - \dfrac{1}{-40} = \dfrac{1}{15}
1v1=115−140=8−3120=124\dfrac{1}{v_1} = \dfrac{1}{15} - \dfrac{1}{40} = \dfrac{8 - 3}{120} = \dfrac{1}{24}
v1=+24 cmv_1 = +24 \text{ cm}
The positive sign indicates that the image is formed on the side opposite to the object, which is away from the mirror.

For the second image (formed after reflection from the mirror):
The distance between the lens and the mirror is 50 cm50 \text{ cm}. The source is 40 cm40 \text{ cm} from the lens, so it is 50−40=10 cm50 - 40 = 10 \text{ cm} from the mirror.
For the converging (concave) mirror, um=−10 cmu_m = -10 \text{ cm} and fm=−10 cmf_m = -10 \text{ cm}.
Using the mirror formula 1vm+1um=1fm\dfrac{1}{v_m} + \dfrac{1}{u_m} = \dfrac{1}{f_m}:
1vm−110=−110⇒1vm=0⇒vm=∞\dfrac{1}{v_m} - \dfrac{1}{10} = -\dfrac{1}{10} \Rightarrow \dfrac{1}{v_m} = 0 \Rightarrow v_m = \infty
The reflected rays become parallel to the principal axis and travel towards the lens.
For the lens, these parallel rays act as an object at infinity (u2=−∞u_2 = -\infty).
Using the lens formula:
1v2−1−∞=115⇒v2=+15 cm\dfrac{1}{v_2} - \dfrac{1}{-\infty} = \dfrac{1}{15} \Rightarrow v_2 = +15 \text{ cm}
Since the rays are coming from the mirror, they pass through the lens and converge on the other side, which is away from the mirror.

Thus, the two images are formed at 15 cm15 \text{ cm} and 24 cm24 \text{ cm} from the lens, both on the side away from the mirror.
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