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 decreae the distance between the eye lens and the retina beyond a limit
D. the eye is not able to decrease the focal length beyond a limit.

Answer

To solve the question of why a normal eye cannot see objects closer than 25 cm, we can 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 responsible for focusing light onto the retina.

2. **Focal Length of the Eye**:
- The eye can adjust its focal length to focus on objects at various distances. However, there is a limit to how much the eye can change its focal length.

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

4. **Analyzing the Options**:
- **Option A**: The focal length of the eye is 25 cm.
- This is incorrect because the focal length is not a fixed distance; it changes depending on the distance of the object.
- **Option B**: The distance of the retina from the eye lens is 25 cm.
- This is also incorrect; the distance from the lens to the retina varies and is not the reason for the near point limitation.
- **Option C**: The eye is not able to decrease the distance between the eye lens and the retina beyond the limit.
- This option is misleading; the distance between the lens and the retina does not change significantly.
- **Option D**: The eye is not able to decrease the focal length beyond a limit.
- This is the correct option. The eye's lens can only adjust to a certain focal length, which limits how close an object can be for clear vision.

5. **Conclusion**:
- A normal eye cannot see objects closer than 25 cm because it cannot decrease its focal length beyond a certain limit, which prevents it from focusing on nearer objects.

### Final Answer:
The correct answer is **Option D**: The eye is not able to decrease the focal length beyond a limit.