Question:
The focal length of a plane mirror is
The focal length of a plane mirror is
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Since $P = 1/f$, the \textbf{Power} of a plane mirror is $1/\infty = 0$. This makes sense because the mirror does not converge or diverge light; it simply redirects it.
Updated On: Apr 24, 2026
- zero
- 1 cm
- 100 cm
- infinite
- 25 cm
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The Correct Option is D
Solution and Explanation
Concept:
Physics - Ray Optics.
Step 1: Understand the geometry of a plane mirror.
A plane mirror can be thought of as a spherical mirror with an infinitely large radius of curvature ($R = \infty$). It is perfectly flat.
Step 2: Apply the relationship between Focal Length and Radius.
For any mirror, the focal length ($f$) is half of the radius of curvature: $$ f = \frac{R}{2} $$
Step 3: Substitute the value for a flat surface.
$$ f = \frac{\infty}{2} = \infty $$
Step 4: Interpret the physical meaning.
Parallel rays striking a plane mirror remain parallel after reflection. They never converge at a "focus" point in front of the mirror, which mathematically corresponds to a focus located at infinity.
Physics - Ray Optics.
Step 1: Understand the geometry of a plane mirror.
A plane mirror can be thought of as a spherical mirror with an infinitely large radius of curvature ($R = \infty$). It is perfectly flat.
Step 2: Apply the relationship between Focal Length and Radius.
For any mirror, the focal length ($f$) is half of the radius of curvature: $$ f = \frac{R}{2} $$
Step 3: Substitute the value for a flat surface.
$$ f = \frac{\infty}{2} = \infty $$
Step 4: Interpret the physical meaning.
Parallel rays striking a plane mirror remain parallel after reflection. They never converge at a "focus" point in front of the mirror, which mathematically corresponds to a focus located at infinity.
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