In a healthy human eye, the focussing is done by the

Options

A. To and fro movement of the eye-lens
B. Changing the curvature of the retina
C. Change in the convexity of the lens through ciliary tubes
D. Change in the refractive index of the eye fluid.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the mechanism of focusing in a healthy human eye. Focusing is the ability of the eye to adjust to see objects clearly at different distances.

2. **Identifying the Components of the Eye**: The main components involved in focusing are the lens, retina, and ciliary muscles. The lens is responsible for bending light to focus it on the retina, which is the screen where the image is formed.

3. **Role of Ciliary Muscles**: The ciliary muscles are attached to the lens of the eye. These muscles can contract or relax, which changes the shape (curvature) of the lens.

4. **Changing Lens Shape**: When the ciliary muscles contract, the lens becomes more spherical (curved), increasing its power and decreasing its focal length, allowing us to focus on nearby objects. Conversely, when the ciliary muscles relax, the lens flattens, decreasing its power and increasing its focal length, allowing us to focus on distant objects.

5. **Conclusion**: In a healthy human eye, the focusing is done by the ciliary muscles, which adjust the shape of the lens to focus light properly on the retina.

**Final Answer**: In a healthy human eye, the focusing is done by the ciliary muscles.

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