## **Core Concept**
The power of a lens is defined as the reciprocal of its focal length in meters. It is measured in diopters (D). A lens with a power of 0.25 diopters has a specific focal length that can be calculated using the formula: Power (D) = 1 / Focal length (m).

## **Why the Correct Answer is Right**
Given that the power of the lens is 0.25 diopters, we can calculate its focal length using the formula: Focal length (m) = 1 / Power (D). Substituting the given power: Focal length (m) = 1 / 0.25 = 4 meters. This matches option , which represents 4 meters.

## **Why Each Wrong Option is Incorrect**
- **Option A:** represents 0.25 meters, which would correspond to a lens power of 1 / 0.25 = 4 diopters, not 0.25 diopters.
- **Option B:** represents 2.5 meters, corresponding to a lens power of 1 / 2.5 = 0.4 diopters, not 0.25 diopters.
- **Option D:** represents 0.4 meters, corresponding to a lens power of 1 / 0.4 = 2.5 diopters, not 0.25 diopters.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that lens power and focal length are inversely related. For ophthalmology or optometry exams, it's crucial to be comfortable with converting between diopters and meters. A simple mnemonic is that a 1 diopter lens has a focal length of 1 meter.

## **Correct Answer:** .