Concepts Of Physics MCQ Edition [Volume 1]PhysicsOptical Instruments
Identify the correct alternatives.
Options
- AIf the near point is situated 1 m away from the eye, a divergent lens should be used.
- BWhen the near point goes ahead, the power of the convergent lens should be reduced.
- CIf the far point is situated 1 m away from the eye, a divergent lens should be used.
- 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 x is greater than the normal near point D = 0.25 m . To correct this, a convergent lens is used. The power P of the lens is given by: P = 1 v - 1 u = 1 -x - 1 -D = 1 D - 1 x If the near point is 1 m , P = 1 0.25 - 1 1 = +3 D . Since the power is positive, a convergent lens is required. Thus, statement (1) is incorrect. When the near point goes ahead, the distance x increases. This causes 1 x to decrease, which increases the value of P = 1 D - 1 x . Therefore,