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A mirror is used to produce an image with magnification of 1/4. If the distance between object and its image is 40 cm, then the focal length of the mirror is

Asked in JEE Main 7th April 2nd Shift 2025 · Mirror formula and magnification

• (1) 15 cm
• (2) 10.7 cm
• (3) 12.7 cm
• (4) 10 cm

Step-by-step solution

Why the other options are wrong

• (1) Takes the separation as |u|-|v| (real image) and then slips in the arithmetic.
• (3) Rounds a wrong object distance; the exact value is (32)/3 cm.
• (4) Uses f=v/(1-m) style shortcut incorrectly, getting a round number.

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