The radius of curvature of a plane mirror of focal length `40 cm` is :

Options

A. 20 cm
B. 40 cm
C. zero
D. None

Explanation

To find the radius of curvature of a plane mirror given its focal length, we can use the relationship between the radius of curvature (R) and the focal length (f). The formula is:

\[ R = 2f \]

### Step-by-Step Solution:

1. **Identify the given focal length**:
The problem states that the focal length (f) of the plane mirror is 40 cm.

2. **Use the formula for radius of curvature**:
We know that for mirrors, the radius of curvature (R) is related to the focal length (f) by the equation:
\[ R = 2f \]

3. **Substitute the value of focal length into the formula**:
Now, we can substitute the given focal length into the formula:
\[ R = 2 \times 40 \, \text{cm} \]

4. **Calculate the radius of curvature**:
Performing the multiplication:
\[ R = 80 \, \text{cm} \]

5. **State the final answer**:
Therefore, the radius of curvature of the plane mirror is 80 cm.

### Final Answer:
The radius of curvature of the plane mirror is **80 cm**.

---