Question:
Focal length of a plane mirror is :
Focal length of a plane mirror is :
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A plane mirror can be thought of as a spherical mirror with an infinitely large radius of curvature. Consequently, its focal length, which is half of the radius of curvature, is also infinite. This is why parallel light rays reflecting off a plane mirror remain parallel.
Updated On: Jun 5, 2025
- Infinite
- Zero
- 1 m
- 5 cm
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The Correct Option is A
Solution and Explanation
Step 1: Understand the concept of focal length.
The focal length of a mirror is the distance from the mirror's pole to its principal focus. For spherical mirrors, it is half of the radius of curvature ($f = R/2$). Step 2: Consider the curvature of a plane mirror.
A plane mirror can be considered as a part of a spherical mirror with an infinitely large radius of curvature. In other words, its reflecting surface is flat, which implies it has no curvature, or equivalently, an infinite radius of curvature. Step 3: Determine the focal length based on curvature.
Since a plane mirror has an infinite radius of curvature ($R = \infty$), its focal length ($f = R/2$) will also be infinite ($f = \infty/2 = \infty$).
Parallel rays of light incident on a plane mirror continue to be parallel after reflection, or appear to diverge from a point at infinity behind the mirror, hence the infinite focal length. Step 4: Compare the conclusion with the given options.
The focal length of a plane mirror is infinite. This matches option (1). $$(1) Infinite$$
The focal length of a mirror is the distance from the mirror's pole to its principal focus. For spherical mirrors, it is half of the radius of curvature ($f = R/2$). Step 2: Consider the curvature of a plane mirror.
A plane mirror can be considered as a part of a spherical mirror with an infinitely large radius of curvature. In other words, its reflecting surface is flat, which implies it has no curvature, or equivalently, an infinite radius of curvature. Step 3: Determine the focal length based on curvature.
Since a plane mirror has an infinite radius of curvature ($R = \infty$), its focal length ($f = R/2$) will also be infinite ($f = \infty/2 = \infty$).
Parallel rays of light incident on a plane mirror continue to be parallel after reflection, or appear to diverge from a point at infinity behind the mirror, hence the infinite focal length. Step 4: Compare the conclusion with the given options.
The focal length of a plane mirror is infinite. This matches option (1). $$(1) Infinite$$
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