Question

For a normal eye, in case of an adult, the least distance of distinct vision is?
A. 5 to 8 cm
B. 10 to 15 cm
C. 20 to 25 cm
D. 30 to 35 cm

Answer 1

For a normal human eye, there is a minimum distance until which it can see any object clearly and distinctly. This minimum distance for an eye to see clearly and distinctly is known as the least distance of distinct vision. Minimum distance depends on the Eye focus and retina power also called eye power.

Complete step-by-step answer:
When we see a distant object, the parallel rays of light falling on our eye are focused by the lens on the retina. Therefore, the object is seen distinctly.
So, when we see a nearby object, the muscles in the eye contract to increase the curvature of the lens. Hence, the focal length of the lens decreases and a clear image of the object is formed at the retina. But there is a limit to this accommodation. The nearest distance up to which the eye can see clearly is called the ‘least distance of distinct vision’ For normal eyes it is 25cm. The point at this distance is called the ‘near point’ of the eye. If an object is placed at a distance less than this, from the eye, it will not be seen distinctly.

Therefore, for a normal eye, in case of an adult, the least distance of distinct vision is 20 to 25 cm.

Hence, (C) option is correct.

Note: It is important to note that this power of changing the focal length of the eye is called ‘power of accommodation’. Moreover, a normal eye can see distinctly an object at infinity, which is called the ‘far point’ of the normal eye.