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

• A. -0.5 D
• B. +0.5 D
• C. +0.25 D
• D. -0.25 D

Hint:

For myopia, power of corrective lens \( P = -100 / \) far point (in cm).

Answer 1

The Correct Option is D

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.