A concave mirror of focal length `f` (in air) is immersed in water `(mu=4//3)`. The focal length of the mirror in water will be

Options

A. f
B. `(4)/(3)`f
C. `(3)/(4)`f
D. `(7)/(3)`f

Explanation

To find the focal length of a concave mirror when it is immersed in water, we can follow these steps:

### Step 1: Understand the Focal Length of a Concave Mirror
The focal length \( f \) of a concave mirror is given by the formula:
\[ f = \frac{R}{2} \]
where \( R \) is the radius of curvature of the mirror. This formula indicates that the focal length is determined by the physical properties of the mirror itself.

### Step 2: Recognize the Effect of Medium on Focal Length
The focal length of a mirror does not change with the refractive index of the surrounding medium. This is a key point because it differentiates mirrors from lenses.

### Step 3: Apply the Concept to the Given Problem
In this problem, the concave mirror has a focal length \( f \) in air. Since the mirror is now immersed in water, we need to determine if the focal length changes.

### Step 4: Conclude the Focal Length in Water
Since the focal length of a concave mirror is independent of the medium it is in, the focal length of the mirror in water will still be:
\[ f_{water} = f \]

### Final Answer
Thus, the focal length of the concave mirror when immersed in water is:
\[ \text{Focal length in water} = f \]