A transparent plastic bag filled with air forms a concave lens. Now, if this bag is completely immersed in water, then it behaves as

Options

A. convergent lens
B. rectangular slab
C. divergent lens
D. equilateral prism

Explanation

To solve the problem, we need to analyze the behavior of a transparent plastic bag filled with air when it is immersed in water. The bag acts as a lens, and we need to determine how it behaves in this new medium.

### Step-by-Step Solution:

1. **Identify the Lens Type**:
- A transparent plastic bag filled with air forms a concave lens when it is in air. This is because the refractive index of air is less than that of the plastic, causing light rays to diverge.

2. **Consider the New Medium**:
- When this bag is completely immersed in water, we need to compare the refractive indices of the materials involved. The refractive index of water is greater than that of air.

3. **Refractive Index Comparison**:
- The refractive index of air (n_air) is approximately 1.00, while the refractive index of water (n_water) is about 1.33. Since n_water > n_air, the surrounding medium (water) is denser than the medium inside the bag (air).

4. **Effect on Lens Behavior**:
- When the lens is surrounded by a denser medium, the behavior of the lens changes. A lens that was previously diverging (concave) in air will now behave as a converging lens when immersed in a denser medium.

5. **Conclusion**:
- Therefore, when the transparent plastic bag filled with air is completely immersed in water, it behaves as a convergent lens.

### Final Answer:
The transparent plastic bag filled with air behaves as a **convergent lens** when completely immersed in water.

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