Question

The change in focal length of an eye lens is caused by the action of :

• A. pupil
• B. retina
• C. ciliary muscle
• D. iris

Hint:

The eye focuses by changing the lens shape. Think of ciliary muscles as the "adjusters":
Ciliary muscles {contract} \(\rightarrow\) Lens gets {thicker} (for {near} objects).
Ciliary muscles {relax} \(\rightarrow\) Lens gets {thinner} (for {far} objects). This change in thickness alters the focal length.

Answer 1

The Correct Option is C

Concept: The human eye can focus on objects at different distances by changing the shape (and thus the focal length) of its lens. This process is called accommodation. Step 1: The Role of the Eye Lens and Accommodation The eye lens is a flexible, convex lens. To form a clear image on the retina for objects at various distances, its converging power (related to focal length) must change. Step 2: How Ciliary Muscles control the Lens The eye lens is suspended by suspensory ligaments which are attached to the ciliary muscles.
For distant objects: The ciliary muscles relax. This causes the suspensory ligaments to pull on the lens, making it thinner and less curved (longer focal length).
For near objects: The ciliary muscles contract. This releases tension on the suspensory ligaments, allowing the elastic lens to become thicker and more curved (shorter focal length). Thus, the ciliary muscles are responsible for changing the focal length of the eye lens. Step 3: Functions of other eye parts mentioned
(1) Pupil: The opening in the iris that lets light in. Its size changes to control light intensity, not focal length.
(2) Retina: The light-sensitive layer at the back of the eye where the image is formed. It detects light but doesn't change the lens shape.
(4) Iris: The colored part of the eye that controls the size of the pupil. Therefore, the ciliary muscles cause the change in focal length.