Correct Option
The correct option is25 cm
Explanation
The human eye functions as a natural optical instrument containing a convex lens. The ability of the eye lens to adjust its focal length to focus on objects at different distances is known as accommodation. However, there is a physical limit to the curvature modification possible by the ciliary muscles.
Detailed Analysis
The least distance of distinct vision (also called the near point) is the minimum distance at which an object can be placed so that a normal eye may see it clearly without strain.
- For a normal adult eye: This distance is standardly defined as 25 cm. If an object is placed closer than this distance, the ciliary muscles cannot sufficiently increase the curvature of the lens to focus the image sharply on the retina, resulting in a blurred image and eye strain.
- Variation with Age: This distance varies with age. It is very short in infants (approximately 7 to 8 cm) and increases with age due to the stiffening of the ciliary muscles and the lens (a condition known as presbyopia).
- Significance in Optics: In the study of optical instruments (such as microscopes and telescopes), this distance is denoted by the symbol D and is used to calculate magnification power.
Therefore, for a standard normal human eye, the value is 25 cm.
Key Takeaway: The standard value for the least distance of distinct vision (near point) for a normal human eye is 25 cm.