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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 mirror M_1 , a point source S , and a diverging mirror...

A converging mirror M1M_1, a point source SS, and a diverging mirror M2M_2 are configured. The source is located at a distance of 30 cm30 \text{ cm} from M1M_1. Each mirror has a focal length of 20 cm20 \text{ cm}. Taking into account only the images produced by at most two reflections, it is observed that one image coincides with the source itself. Determine the location of the image formed by a single reflection from M2M_2.

Options

  1. A20 cm20 \text{ cm} from the diverging mirror closer to the converging mirror
  2. B20 cm20 \text{ cm} from the diverging mirror farther from the converging mirror
  3. C10 cm10 \text{ cm} from the diverging mirror farther from the converging mirror
  4. D10 cm10 \text{ cm} from the diverging mirror closer to the converging mirror

Correct answer

C. 10 cm10 \text{ cm} from the diverging mirror farther from the converging mirror

Step-by-step solution

For the converging mirror M1M_1, the focal length is f1=−20 cmf_1 = -20 \text{ cm} and the object distance is u1=−30 cmu_1 = -30 \text{ cm}.

Using the mirror formula for M1M_1:
1v1+1u1=1f1\dfrac{1}{v_1} + \dfrac{1}{u_1} = \dfrac{1}{f_1}
1v1−130=−120⇒v1=−60 cm\dfrac{1}{v_1} - \dfrac{1}{30} = -\dfrac{1}{20} \Rightarrow v_1 = -60 \text{ cm}

Let the distance between the source SS and the diverging mirror M2M_2 be xx. The distance between the two mirrors is d=30+xd = 30 + x.

For the image to coincide with the source after two reflections (first from M1M_1 and then from M2M_2), the image I1I_1 formed by M1M_1 acts as a virtual object for M2M_2.

The distance of I1I_1 from M2M_2 is 60−d=60−(30+x)=30−x60 - d = 60 - (30 + x) = 30 - x. Since it is formed behind M2M_2, the object distance for M2M_2 is u2=+(30−x)u_2 = +(30 - x).

The final image is formed at the source SS, which is at a distance xx in front of M2M_2. Thus, the image distance for M2M_2 is v2=−xv_2 = -x.

Using the mirror formula for M2M_2 (f2=+20 cmf_2 = +20 \text{ cm}):
1v2+1u2=1f2\dfrac{1}{v_2} + \dfrac{1}{u_2} = \dfrac{1}{f_2}
1−x+130−x=120\dfrac{1}{-x} + \dfrac{1}{30 - x} = \dfrac{1}{20}
−30+x+x−x(30−x)=120\dfrac{-30 + x + x}{-x(30 - x)} = \dfrac{1}{20}
2x−30x2−30x=−120\dfrac{2x - 30}{x^2 - 30x} = -\dfrac{1}{20}
−40x+600=x2−30x-40x + 600 = x^2 - 30x
x2+10x−600=0x^2 + 10x - 600 = 0
(x+30)(x−20)=0(x + 30)(x - 20) = 0

Since distance cannot be negative, x=20 cmx = 20 \text{ cm}.

To find the location of the image formed by a single reflection from M2M_2, we use u=−20 cmu = -20 \text{ cm} and f=+20 cmf = +20 \text{ cm}:
1v+1−20=120\dfrac{1}{v} + \dfrac{1}{-20} = \dfrac{1}{20}
1v=120+120=110⇒v=+10 cm\dfrac{1}{v} = \dfrac{1}{20} + \dfrac{1}{20} = \dfrac{1}{10} \Rightarrow v = +10 \text{ cm}

The positive sign indicates that the image is formed 10 cm10 \text{ cm} behind the diverging mirror M2M_2. Since M1M_1 is in front of M2M_2, the region behind M2M_2 is farther from the converging mirror.

Answer: 10 cm10 \text{ cm} from the diverging mirror farther from the converging mirror
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