Assertion : A convex lens of glass `(mu = 1.5)` behave as a diverging lens when immersed in carbon disulphinde of higher refractive index `(mu = 1.65)`.
Reason : A diverging lens is thinner in the middle and thicker at the edges.

Options

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

Explanation

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

### Step 1: Understanding the Assertion
The assertion states that a convex lens of glass (with a refractive index, μ = 1.5) behaves as a diverging lens when immersed in carbon disulphide (with a higher refractive index, μ = 1.65).

### Step 2: Analyzing the Lens Maker's Formula
To understand how the lens behaves when immersed in a medium with a higher refractive index, we can use the lens maker's formula:
\[ \frac{1}{F} = \left(\frac{n_g}{n_c} - 1\right) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \]
Where:
- \( n_g \) = refractive index of the lens material (1.5 for glass)
- \( n_c \) = refractive index of the surrounding medium (1.65 for carbon disulphide)
- \( R_1 \) and \( R_2 \) are the radii of curvature of the lens surfaces.

### Step 3: Substituting Values
Substituting the values into the formula:
\[ \frac{1}{F} = \left(\frac{1.5}{1.65} - 1\right) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \]
Calculating \( \frac{1.5}{1.65} \):
\[ \frac{1.5}{1.65} \approx 0.909 \]
Thus,
\[ \frac{1}{F} = (0.909 - 1) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) = (-0.091) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \]
Since \( \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \) is positive for a convex lens (as \( R_1 > R_2 \)), we see that \( \frac{1}{F} \) becomes negative.

### Step 4: Conclusion about Focal Length
A negative value of \( \frac{1}{F} \) indicates that the focal length \( F \) is also negative. This means that the lens behaves as a diverging lens when immersed in a medium with a higher refractive index.

### Step 5: Analyzing the Reason
The reason states that a diverging lens is thinner in the middle and thicker at the edges. While this is true for a concave lens, it does not explain why a convex lens behaves as a diverging lens when immersed in a medium of higher refractive index.

### Final Conclusion
- The assertion is true: A convex lens behaves as a diverging lens when immersed in carbon disulphide.
- The reason is not a correct explanation of the assertion.

Thus, the correct answer is that the assertion is true, but the reason is false.