Power of a lens of focal length f is given by?

Appeared in: RRB Nsg. Superintendent-20 July 2019 (Shift-2)

Explanation

The power of a lens (P) is defined as the reciprocal of its focal length (f).
The mathematical formula is P = 1/f.
The unit of power is the Diopter (D), which is used when the focal length is measured in meters (m).
A shorter focal length results in a higher power, as the lens bends light more strongly.

Option A: This formula is incorrect. The variable 'u' typically represents the object distance in the lens maker's formula, not a component of lens power.
Option C: This is not a standard formula in optics and does not correctly relate power and focal length.
Option D: This is incorrect. Power is inversely proportional to the focal length (1/f), not directly equal to it.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Relationship between lens power and focal length as background academic context rather than a clinical decision trigger.
Understanding lens power is fundamental for optometrists and ophthalmologists when prescribing corrective eyeglasses or contact lenses.
A prescription of +2.0 D means a convex lens with a focal length of 0.5 meters is needed to correct hyperopia (farsightedness).
A prescription of -3.0 D means a concave lens with a focal length of -0.33 meters is needed to correct myopia (nearsightedness).

How to Approach the Question

This is a direct recall question based on a fundamental physics definition.
Identify the key terms: 'Power of a lens' (P) and 'focal length' (f).
Recall the definition of lens power. It is defined as the ability of a lens to converge or diverge light, which is the inverse of its focal length.
Evaluate the given options to find the one that matches the formula P = 1/f.
Eliminate other options that use incorrect variables (like 'u' for object distance) or incorrect relationships.

Concept Tested & Keywords

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E6 Physiology Guyton 4SAE Part 3 pp. 139-141, 140-142

Practise the full RRB Nsg. Superintendent-20 July 2019 (Shift-2)

Attempt every question from this paper in a timed mock, then review the full solution for each one.