Question:
Which of the following mirrors has a positive focal length?
Which of the following mirrors has a positive focal length?
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Concave mirrors are used in torches, headlights, and solar cookers because they converge light at a real focus.
Updated On: Sep 19, 2025
- Convex mirror
- Plane mirror
- Concave mirror
- None of these
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The Correct Option is C
Solution and Explanation
Step 1: Conceptual Understanding:
The focal length ($f$) of a mirror is the distance between the pole and the focus. The sign convention determines whether $f$ is positive or negative.
Concave mirror: Focus is in front of the mirror → focal length is positive.
Convex mirror: Focus lies behind the mirror → focal length is negative.
Plane mirror: Focal length is considered infinite.
Step 2: Application:
Concave mirrors converge parallel rays to a real focus in front of them. Thus, according to the sign convention, their focal length is positive. Step 3: Final Answer:
The mirror with positive focal length is the Concave mirror.
The focal length ($f$) of a mirror is the distance between the pole and the focus. The sign convention determines whether $f$ is positive or negative.
Concave mirror: Focus is in front of the mirror → focal length is positive.
Convex mirror: Focus lies behind the mirror → focal length is negative.
Plane mirror: Focal length is considered infinite.
Step 2: Application:
Concave mirrors converge parallel rays to a real focus in front of them. Thus, according to the sign convention, their focal length is positive. Step 3: Final Answer:
The mirror with positive focal length is the Concave mirror.
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