Question

The power of accommodation for the normal eye is.
A. 4 D
B. 40 D
C. 44 D
D. 400 D

Answer 1

Hint: First we need to know the normal vision length or the focal length of the eye, then we need to know the formula of power for a lens. When we apply this focal length of the eye into the formula for power we get the required result.

Formula used:
The power of a lens is given by:
$P=\dfrac{1}{f}$
where f represents the focal length of the lens.

Complete step by step answer:
If the object that is in consideration is kept at a distance, then the focal length has to be large enough, to form the image at the retina. Here, in this case, the ciliary muscles relax and thereby thinning the eye lens. On thinning the eye lens the focal length increases and the image formed falls perfectly on the retina. Similarly, in the case when the object is near, the ciliary muscles contract as a result of which the lens thickens. This thickening of the lens causes a reduction in the focal length for ideal image formation.
We know that our normal vision length or the focal length of the eye is 25 cm or 0.25 m.
The formula for the power of a lens is inversely proportional to the focal length of the lens,
$P=\dfrac{1}{f}$
$P=\dfrac{1}{0.25}$,
$P=4D$
Therefore, the Power of accommodation is 4 D (dioptre), for a human of normal eyesight.

Therefore option A is the correct option.

Additional Information:
Accommodation of the eye is the process by which certain muscles in our eyes known as ciliary muscles change the focal length of the eyes so that the image is clearly formed on the retina. This process always varies for distant and near objects and also for objects towards the eye or moving away from the eye. By adjusting the focal length of the eye, the eye is actually changing its lens power. This is known as the accommodating power of the eye.

Note:
If the focal length of a lens is negative then the lens is considered as a concave lens, and if the focal length of the lens is positive then the lens is considered as a concave lens.