The minimum focal length of the eye-lens of a healthy human being is
2.7 cm
- 2·5 cm
25 cm
- 1 cm
The Correct Option is C
Approach Solution - 1
The human eye lens has the ability to change its focal length to focus on objects at various distances. This ability to adjust is known as accommodation. The minimum focal length of the eye lens when focusing on a nearby object is crucial for clear vision. The typical range of focal lengths for a healthy human eye lens extends from about 2.7 cm to 2.5 cm when observing distant objects, and can be reduced significantly when focusing on closer objects. However, the minimum focal length corresponding to the standard near point, which is the closest distance for clear vision without strain, is approximately 25 cm.
Therefore, the correct answer is 25 cm as it represents the minimum focal length of the eye-lens for a healthy human eye when focusing on nearby objects, such as reading a book held comfortably.
Approach Solution -2
The focal length of the eye-lens varies depending on the object’s distance due to the process called accommodation. In a healthy human eye, when viewing the nearest object clearly (at the near point), the focal length is the minimum. The near point for a healthy human being is about 25 cm, but the lens itself changes shape to focus that close.
In such a condition, the focal length of the eye-lens becomes approximately:
\( f \approx 2.5 \, \text{cm} \)
This is the minimum focal length the eye-lens can attain when fully accommodating.
Final Answer: 2·5 cm
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