Question

Focal length of the eye lens is changed by?
A) Pupil
B) Iris
C) Cornea
D) Ciliary body

Answer 1

Hint The curvature of the eye lens is responsible for the focal length of the eye. The curvature of the lens is controlled by one of the muscles present in the eye which causes the change in focal length.

Complete step by step answer
To see any object, the image must fall precisely on the retina to produce a clear vision. Because the lens-to-retina distance does not change inside the eye, the image distance must be the same for objects at all distances and hence the focal length must be changed so that the image of the object falls on the retina for arbitrary object distance.
The ciliary muscles adjust the shape of the eye and hence the radius of the curvature lens for focusing on nearby or far objects. By changing the shape of the eye lens, the eye changes the focal length of the lens. This mechanism of the eye is called accommodation.
The pupil is the radial opening of the eye which controls the intensity of the light that enters the eye but not its focal length so it is not the correct choice.
The iris is a flat and ring-shaped membrane behind the cornea of the eye with an adjustable pupil and it controls the size of the pupil so it is also not the correct choice.
The cornea is the lens in the eye which helps in focusing the image of the object on the retina but its focal length is changed by the ciliary muscles so option (D) is the correct choice.

Note
While the cornea acts as a lens and focuses the image on the retina, it is the ciliary muscles that change the focal length of the eye. Even by changing the focal length, the eye in a relaxed position can only see clearly at a minimum distance of 25 cm up to a maximum of infinite distance.