Focal Length
The focal length is the distance between the pole of a mirror and its principal focus.
Practice This ConceptMain explanation
Teacher explanation
For a concave mirror, parallel rays meet at the focus after reflection. For a convex mirror, the reflected rays appear to come from the focus behind the mirror. The focal length helps locate images and compare mirror strength.
Example
If the focus is 12 cm from the pole, the focal length is 12 cm.
Simple analogy
Pole to focus gives focal length.
Common confusion
Students sometimes measure from the centre of curvature or from the mirror edge instead of from the pole.
Exam tip
If the question gives radius of curvature, remember f = R/2 for a spherical mirror.
Answer writing and exam use
1-mark use
Write the exact meaning of focal length in one clean line.
2-mark use
Define focal length and add one example or condition.
3-mark use
Explain focal length, show the method or example, and mention the common mistake.
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