I keep getting “Lens/mirror formula & magnification” questions wrong in Ray Optics & Optical Instruments. What's the trap?
ignoring the sign convention, which flips the meaning of the answer.
The usual trap: ignoring the sign convention, which flips the meaning of the answer. How to avoid it: stick to one sign convention (distances measured from the pole) throughout. Go back to the definition — 1/v − 1/u = 1/f links object, image and focal length; magnification m = v/u tells size and orientation.
Key point
Lens/mirror formula & magnification
Common mistake
ignoring the sign convention, which flips the meaning of the answer.
Memory tip
stick to one sign convention (distances measured from the pole) throughout.
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