A convex lens, made of glass, is immersed in water. As a result, its focal length will:

Options

A. increase
B. decrease
C. double
D. remain same

Explanation

To solve the problem regarding the change in the focal length of a convex lens made of glass when it is immersed in water, we can follow these steps:

### Step 1: Understand the Lens Maker's Formula
The lens maker's formula is given by:
\[ \frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
where:
- \( f \) is the focal length of the lens,
- \( \mu \) is the refractive index of the lens material,
- \( R_1 \) and \( R_2 \) are the radii of curvature of the lens surfaces.

### Step 2: Identify the Refractive Indices
For a convex lens made of glass:
- The refractive index of glass, \( \mu_{glass} \), is approximately 1.5.
- The refractive index of water, \( \mu_{water} \), is approximately 1.3.

### Step 3: Calculate the Relative Refractive Index
When the lens is immersed in water, the effective refractive index of the lens with respect to water can be calculated as:
\[ \mu_{relative} = \frac{\mu_{glass}}{\mu_{water}} = \frac{1.5}{1.3} \approx 1.15 \]

### Step 4: Analyze the Effect on Focal Length
From the lens maker's formula, we see that the focal length \( f \) is inversely proportional to \( (\mu - 1) \). Therefore, if the refractive index decreases, the term \( (\mu - 1) \) will also decrease, leading to an increase in the focal length \( f \).

### Step 5: Conclusion
Since the refractive index of the lens decreases from 1.5 to approximately 1.15 when immersed in water, the focal length of the convex lens will increase.

Thus, the answer is: **The focal length will increase.**

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