The change of focal length of an eye lens is caused by the action of the ciliary muscles.

Explanation:
The eye consists of various structures that work together to enable vision. The lens of the eye is responsible for focusing light onto the retina, which then sends signals to the brain for image processing. The ability to change the focal length of the lens is crucial for clear vision at different distances.

Role of the Ciliary Muscles:
The ciliary muscles are located in the ciliary body, which is a ring-shaped structure behind the iris. These muscles play a significant role in adjusting the shape and curvature of the lens, thereby altering its focal length. When the ciliary muscles contract, they exert a pulling force on the lens, causing it to become more rounded and increasing its refractive power. This allows the eye to focus on nearby objects.

Process of Accommodation:
The adjustment of the lens to focus on objects at different distances is called accommodation. When we look at distant objects, the ciliary muscles are relaxed, and the lens is flattened. This reduces its refractive power, making it easier to focus on distant objects. However, when we shift our focus to a nearby object, the ciliary muscles contract, causing the lens to become thicker and more curved. This increases its refractive power, enabling clear vision of nearby objects.

Importance of Ciliary Muscle Action:
The action of the ciliary muscles is crucial for maintaining clear vision at different distances. Without the ability to change the shape of the lens, the eye would have a fixed focal length and would only be able to focus on objects at a specific distance. The action of the ciliary muscles allows the eye to dynamically adjust its focal length, providing clear vision at various distances.

In summary, the ciliary muscles are responsible for changing the shape and curvature of the lens, which in turn alters its focal length. This adjustment, known as accommodation, allows the eye to focus on objects at different distances, providing clear vision.