Question

The change in Focal length of an eye lens is caused by the action of
A. Pupil
B. Retina
C. Ciliary muscles
D. Iris

Answer 1

Hint: The relaxation or contraction of ciliary muscles changes the curvature of the eye lens.
And the focal length of an eye lens is directly connected with Focal length.

Complete step-by-step answer:
The Focal length of an optical system like (eye) is a capability of how strongly the system converges or diverges light.
Focal length of eye change means the curvature of the eye lens got changed. It changed due to the action of ciliary muscles. The Relaxation or Contraction of ciliary muscles changes the curvature of the eye lens.
And this ability of the eye lens to adjust the Focal length of eye lens to see the distinct or nearby objects clearly.
Ciliary muscles are the ring type smooth muscle that supports the eye lens and helps in Accommodation is the process by which the eye changes its focal length and optical power to maintain a clear image, or focus on an object as its distance varies.
Focal length of Human eye is nearly (approximately) 22 mm.

Note: Focal length of eye change means the curvature of the eye lens got charged. It is possible due to relaxation and contraction of the eye lens. When muscles get relaxed the lens becomes faster and when ciliary muscles contract the Eye lens becomes thicker. So it's all happened due to the action of ciliary muscles.