Mirror Formula
The mirror formula relates object distance, image distance, and focal length for spherical mirrors.
Practice This ConceptMain explanation
Teacher explanation
Using the Cartesian sign convention, 1/f = 1/v + 1/u. This formula helps calculate where the image forms and whether it is real or virtual. Correct signs are essential because the same mirror can give very different images depending on object position.
Example
A concave mirror with f = -15 cm and u = -30 cm gives v = -30 cm.
Simple analogy
Formula is short; sign convention is the real test.
Common confusion
Students often insert all distances as positive numbers and then get the wrong image position.
Exam tip
Write the sign convention before starting the calculation; it saves many mistakes.
Answer writing and exam use
1-mark use
Write the exact meaning of mirror formula in one clean line.
2-mark use
Define mirror formula and add one example or condition.
3-mark use
Explain mirror formula, show the method or example, and mention the common mistake.
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