Question

The least distance of distinct vision for a normal person is:
$a){\text{ }}25m$
$b){\text{ }}2.5cm$
$c){\text{ }}25cm$
$d){\text{ }}2.5m$

Answer 1

The human eye can differentiate about $10$ million colors. The eye has many parts that will work jointly to produce clear vision. The normal human eye can focus a proper image of an object on the eye if the object is positioned somewhere from infinity to a certain location called the Near Point.

Complete step by step solution:
The white part of the eye is called sclera shields the eyeball. The pupil or black dot at the middle of the eye is an aperture from which light can enter the eye.

1. The ciliary muscle held along with the eye lens helps the eye modify its focal distance by changing radii of curvature of the eye lens.
2. When the eye is concentrating on a remote object, the ciliary muscles are calm so that the focal distance of the eye lens has its extreme value which is equal to its distance from the retina.
3. The functioning of ciliary muscles in the eye changes from person to person.
4. So, the values of least distance of distinct vision and angle of vision vary with person and age. The very minimum distance at which any objects can be observed most clearly without any strain is called the least distance of distinct vision. It is also termed the near point of the eye. For an adult with a normal vision, we can say that the near point is $25cm$
   The minimum distance from the eye at which an object appears as discrete and distinct is called the least distance of distinct vision or near point. It is about $25cm$ for the human eye. Hence option C is correct.

Note: For a person with $20/20$ vision, the least distance of distinct vision (or LDDV) or the reference seeing distance (or RSD) is minimal and he can comfortably look at something. The range of vision for a well-functioning human eye is in the middle of its near point and far point i.e., from $25cm$ to infinity. If we decrease the distance between the object and eye than this, then the vision would become blurry and the person would experience a headache.