Assertion : If a convex lens of glass is immersed in water its power decreases.
Reason : In water it behaves as a concave lens.

Options

A. If both assertion and reason are true and\ reason is a correct explanation of the assertion
B. If both assertion and reason are true but the reason is not a correct explanation of assertion.
C. If assertion is true but reason is false.
D. If assertion is false but reason is true

Answer

To solve the given question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that if a convex lens of glass is immersed in water, its power decreases.

- The power \( P \) of a lens is given by the formula:
\[ P = \frac{1}{f} \]
where \( f \) is the focal length of the lens. The focal length is related to the refractive indices of the lens material and the surrounding medium by the lens maker's formula:
\[ \frac{1}{f} = (n_l - n_m) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
where \( n_l \) is the refractive index of the lens, \( n_m \) is the refractive index of the medium, and \( R_1 \) and \( R_2 \) are the radii of curvature of the lens surfaces.

- When the lens is in air, the refractive index of air is approximately 1, and for glass, it is greater than 1 (typically around 1.5). When the lens is immersed in water (with a refractive index of approximately 1.33), the effective refractive index difference \( (n_l - n_m) \) decreases.

- Thus, as the refractive index of the medium increases (from air to water), the power of the lens decreases because the focal length increases. Therefore, the assertion is **true**.

### Step 2: Analyze the Reason
The reason states that in water, the lens behaves as a concave lens.

- A lens behaves as a concave lens when the refractive index of the lens material is less than that of the surrounding medium. In this case, since the refractive index of glass (the lens) is greater than that of water, the lens will still behave as a convex lens, even though its power decreases.

- Therefore, the reason is **false**.

### Conclusion
- The assertion is true: If a convex lens of glass is immersed in water, its power decreases.
- The reason is false: In water, it does not behave as a concave lens; it remains a convex lens.

### Final Answer
The correct answer is that the assertion is true, but the reason is false.

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