In human eye the focussing is done by

Options

A. to and fro movement of eye lens
B. to and fro movement of the retina
C. change in the convexity of the lens surface
D. change in the refractive index of the eye fluids

Answer

To solve the question "In human eye the focusing is done by," we need to analyze the options provided and understand the functioning of the human eye.

### Step-by-Step Solution:

1. **Understanding the Structure of the Eye**:
- The human eye consists of several parts, including the cornea, lens, and retina. The lens is a convex lens that helps in focusing light onto the retina.

2. **Identifying the Options**:
- The options provided are:
a) To and fro movement of eye lens
b) To and fro movement of retina
c) Change in the convexity of lens surface
d) Change in the refractive index of eye fluids

3. **Analyzing Each Option**:
- **Option a**: To and fro movement of eye lens - This is incorrect as the lens does not move back and forth; its position is fixed.
- **Option b**: To and fro movement of retina - This is also incorrect; the retina is fixed in place and does not move.
- **Option c**: Change in the convexity of lens surface - This is correct. The lens changes its shape (becomes more or less convex) to focus on objects at different distances.
- **Option d**: Change in the refractive index of eye fluids - While the refractive index plays a role in focusing, the primary mechanism for focusing is the change in the lens shape.

4. **Conclusion**:
- The correct answer is **Option c: Change in the convexity of lens surface**. This change allows the eye to focus on objects at varying distances, ensuring that the image is formed on the retina.