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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Optical Instruments

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Optical Instruments

A compound microscope is composed of an objective with a focal...

A compound microscope is composed of an objective with a focal length of 1 cm1 \text{ cm} and an eyepiece with a focal length of 5 cm5 \text{ cm}. An object is positioned 0.5 cm0.5 \text{ cm} from the objective. Determine the separation between the lenses required for the microscope to project an inverted real image of the object onto a screen located 30 cm30 \text{ cm} behind the eyepiece.

Options

  1. A4 cm4 \text{ cm}
  2. B6 cm6 \text{ cm}
  3. C5 cm5 \text{ cm}
  4. D7 cm7 \text{ cm}

Correct answer

C. 5 cm5 \text{ cm}

Step-by-step solution

Using the lens formula for the objective lens:

1vo−1uo=1fo\dfrac{1}{v_o} - \dfrac{1}{u_o} = \dfrac{1}{f_o}

Substituting uo=−0.5 cmu_o = -0.5 \text{ cm} and fo=1 cmf_o = 1 \text{ cm}:

1vo−1−0.5=11\dfrac{1}{v_o} - \dfrac{1}{-0.5} = \dfrac{1}{1}

1vo+2=1⇒1vo=−1⇒vo=−1 cm\dfrac{1}{v_o} + 2 = 1 \Rightarrow \dfrac{1}{v_o} = -1 \Rightarrow v_o = -1 \text{ cm}

The image formed by the objective is virtual and is located 1 cm1 \text{ cm} in front of the objective.

For the eyepiece, the final image is real and is formed on a screen 30 cm30 \text{ cm} behind it. Using the lens formula for the eyepiece:

1ve−1ue=1fe\dfrac{1}{v_e} - \dfrac{1}{u_e} = \dfrac{1}{f_e}

Substituting ve=30 cmv_e = 30 \text{ cm} and fe=5 cmf_e = 5 \text{ cm}:

130−1ue=15\dfrac{1}{30} - \dfrac{1}{u_e} = \dfrac{1}{5}

1ue=130−15=1−630=−530=−16\dfrac{1}{u_e} = \dfrac{1}{30} - \dfrac{1}{5} = \dfrac{1 - 6}{30} = -\dfrac{5}{30} = -\dfrac{1}{6}

ue=−6 cmu_e = -6 \text{ cm}

The intermediate image acts as the object for the eyepiece and is located 6 cm6 \text{ cm} in front of it.

Let the separation between the lenses be LL. Since the intermediate image is 1 cm1 \text{ cm} in front of the objective, its total distance from the eyepiece is L+1 cmL + 1 \text{ cm}.

Therefore, L+1=6⇒L=5 cmL + 1 = 6 \Rightarrow L = 5 \text{ cm}.
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