A normal eye is not able to see objects closer than 25 cm because

Options

A. the focal length of the eye is 25 cm
B. the distance of the retina from the eye lens is 25 cm
C. the eye is not able to increase the focal length beyond a limit
D. the eye is not able to decrease the focal length beyond a limit

Explanation

To solve the question, "A normal eye is not able to see objects closer than 25 cm because," we will analyze how the human eye functions and the limitations it has regarding focal length.

### Step-by-Step Solution:

1. **Understanding Eye Structure**:
- The human eye consists of a lens and a retina. The lens focuses light onto the retina, where the image is formed.

2. **Image Formation**:
- When an object is at a distance (u), light rays from the object enter the eye and are refracted by the lens to form an image on the retina (at distance V).

3. **Lens Formula**:
- The relationship between object distance (u), image distance (V), and focal length (F) of the lens is given by the lens formula:
\[ \frac{1}{V} = \frac{1}{F} - \frac{1}{u} \]

4. **Limit of Focal Length**:
- The eye can adjust its focal length to focus on objects at varying distances. However, there is a limit to how much the focal length can be decreased.

5. **Near Point of the Eye**:
- The closest distance at which the eye can focus on an object is known as the near point. For a normal eye, this is typically around 25 cm.

6. **Conclusion**:
- Since the eye cannot decrease its focal length beyond a certain limit (which is approximately 25 cm), it cannot focus on objects closer than this distance.

### Final Answer:
The correct reason why a normal eye is not able to see objects closer than 25 cm is: **The eye is not able to decrease the focal length beyond a limit.**

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