An under-water swimmer cannot see very clearly even in absolutely clear water because of

Options

A. absorption of light in water
B. scattering of light in water
C. reduction of speed of light in water
D. change in the focal length of eye lens

Explanation

To solve the question, "An underwater swimmer cannot see very clearly even in absolutely clear water because of," we need to analyze the situation involving light refraction and the human eye.

### Step-by-Step Solution:

1. **Understanding Refraction**: When light travels from one medium to another (like air to water), it bends due to a change in speed. This bending of light is known as refraction.

2. **Human Eye as a Lens**: The human eye functions similarly to a convex lens, which focuses light onto the retina. The eye lens has a specific focal length that allows it to focus light coming from objects at various distances.

3. **Change of Medium**: When a swimmer is underwater, the surrounding medium changes from air (where the eye is adapted to focus) to water. Water has a different refractive index compared to air.

4. **Effect on Focal Length**: The focal length of the eye lens is optimized for air. When submerged in water, the effective focal length of the eye changes because the light rays are refracted differently in water than they are in air.

5. **Resulting Blurriness**: Due to this change in focal length, the eye cannot focus light correctly on the retina when looking at objects underwater. This results in a blurred image, making it difficult for the swimmer to see clearly.

6. **Conclusion**: Therefore, the inability of an underwater swimmer to see clearly, even in clear water, is primarily due to the change in the focal length of the eye lens caused by the different refractive index of water compared to air.

### Final Answer:
An underwater swimmer cannot see very clearly even in absolutely clear water because the focal length of the eye lens changes due to the different refractive index of water compared to air.

---