If a concave mirror is held in water, then its focal length

Options

A. increases
B. decreases
C. remains unchanged
D. may increase or decrease depending upon the level of water

Answer

To determine the effect of placing a concave mirror in water on its focal length, we can follow these steps:

### Step 1: Understand the nature of a concave mirror
A concave mirror is a spherical mirror that curves inward. 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.

### Step 2: Recognize the principle of reflection
The focal length of a mirror is determined by its shape and does not depend on the medium in which it is placed. This is because the focal length is derived from the geometry of the mirror and the law of reflection, which states that the angle of incidence equals the angle of reflection.

### Step 3: Consider the effect of the surrounding medium
While the refractive index of the surrounding medium (in this case, water) affects the focal length of lenses (due to refraction), it does not affect the focal length of mirrors. This is because mirrors operate based on reflection, not refraction.

### Step 4: Conclude the effect on focal length
Since the focal length of a concave mirror is independent of the surrounding medium, placing the concave mirror in water will not change its focal length. Therefore, the focal length remains unchanged.

### Final Answer
The focal length of the concave mirror when held in water is unchanged.

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