MCQ
The focal length of a plane mirror is ______.
- Azero
- ✓infinity
- Cequal to half the object distance
- Dequal to the object distance
✓
Answer
Correct option: B.
infinity
(B) infinity
Hint : In case of plane mirror $|v|=|u|$, also according to the New Cartesian sign convention, $v$ is positive and $u$ is negative.
Now, from the mirror formula $\frac{1}{f}=\frac{1}{u}+\frac{1}{v},$ for a plane mirror, as $|v|=|u|$ and $u$ is negative while $v$ is positive, we get $\frac{1}{f}=0 \quad \therefore f=\infty$ (infinity)
Hint : In case of plane mirror $|v|=|u|$, also according to the New Cartesian sign convention, $v$ is positive and $u$ is negative.
Now, from the mirror formula $\frac{1}{f}=\frac{1}{u}+\frac{1}{v},$ for a plane mirror, as $|v|=|u|$ and $u$ is negative while $v$ is positive, we get $\frac{1}{f}=0 \quad \therefore f=\infty$ (infinity)
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