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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

Identify the correct alternatives.

Identify the correct alternatives.

Options

  1. AIf the near point is situated 1 m1 \text{ m} away from the eye, a divergent lens should be used.
  2. BWhen the near point goes ahead, the power of the convergent lens should be reduced.
  3. CIf the far point is situated 1 m1 \text{ m} away from the eye, a divergent lens should be used.
  4. DWhen the far point goes ahead, the power of the divergent lens should be reduced.

Correct answer

D. When the far point goes ahead, the power of the divergent lens should be reduced.

Step-by-step solution

For an eye suffering from hypermetropia, the near point xx is greater than the normal near point D=0.25 mD = 0.25 \text{ m}. To correct this, a convergent lens is used. The power PP of the lens is given by:

P=1v−1u=1−x−1−D=1D−1xP = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-x} - \dfrac{1}{-D} = \dfrac{1}{D} - \dfrac{1}{x}

If the near point is 1 m1 \text{ m}, P=10.25−11=+3 DP = \dfrac{1}{0.25} - \dfrac{1}{1} = +3 \text{ D}. Since the power is positive, a convergent lens is required. Thus, statement (1) is incorrect.

When the near point goes ahead, the distance xx increases. This causes 1x\dfrac{1}{x} to decrease, which increases the value of P=1D−1xP = \dfrac{1}{D} - \dfrac{1}{x}. Therefore, the power of the convergent lens should be increased. Thus, statement (2) is incorrect.

For an eye suffering from myopia, the far point yy is at a finite distance instead of infinity. To correct this, a divergent lens is used. The power PP of the lens is given by:

P=1v−1u=1−y−1−∞=−1yP = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-y} - \dfrac{1}{-\infty} = -\dfrac{1}{y}

If the far point is 1 m1 \text{ m}, P=−11=−1 DP = -\dfrac{1}{1} = -1 \text{ D}. Since the power is negative, a divergent lens is required. Thus, statement (3) is correct.

When the far point goes ahead, the distance yy increases. The magnitude of the power of the divergent lens is ∣P∣=1y|P| = \dfrac{1}{y}. As yy increases, ∣P∣|P| decreases. Therefore, the power of the divergent lens should be reduced. Thus, statement (4) is correct.
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