Question:
If the far point of a short sighted person is 400 cm, then the power of the lens required to enable him to see very distant objects clearly is
If the far point of a short sighted person is 400 cm, then the power of the lens required to enable him to see very distant objects clearly is
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For myopia, power of corrective lens \( P = -100 / \) far point (in cm).
Updated On: Mar 18, 2026
- -0.5 D
- +0.5 D
- +0.25 D
- -0.25 D
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The Correct Option is D
Solution and Explanation
Step 1: Understand far point
The far point is the maximum distance up to which a person can see clearly without strain. For a short-sighted person, far point \( D_f = 400\, cm = 4\, m \).
Step 2: Calculate power of lens
The power \( P \) in diopters for correcting myopia is given by: \[ P = -\frac{100}{D_f (\mathrm{cm})} = -\frac{100}{400} = -0.25 \, D \] Step 3: Significance of negative power
Negative power indicates a diverging (concave) lens is required for correction. Step 4: Conclusion
The required lens power is -0.25 D.
The far point is the maximum distance up to which a person can see clearly without strain. For a short-sighted person, far point \( D_f = 400\, cm = 4\, m \).
Step 2: Calculate power of lens
The power \( P \) in diopters for correcting myopia is given by: \[ P = -\frac{100}{D_f (\mathrm{cm})} = -\frac{100}{400} = -0.25 \, D \] Step 3: Significance of negative power
Negative power indicates a diverging (concave) lens is required for correction. Step 4: Conclusion
The required lens power is -0.25 D.
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