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JEE Main · 2026 · 22 Jan 2026 · Shift 2 · Geometrical Optics

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In parallax method for the determination of focal length of a concave mirror, the object should always be placed :
  1. (A)between the focus (F) and the centre of curvature (C) of the mirror ONLY
  2. (B)at any point beyond the focus (F) of the mirror✓ Correct
  3. (C)beyond the centre of the curvature (C) of the mirror ONLY
  4. (D)between the pole (P) and the focus (F) of the concave mirror ONLY

Answer: (B)

Solution

The parallax method locates a real image by matching it against a reference pin, so a real image must be formed. A concave mirror produces a real image only when the object is placed beyond the focus; with the object inside the focus the image is virtual and cannot be located by parallax. Hence the object should always be placed at any point beyond the focus of the mirror.

JEE Main · 2026 · 22 Jan 2026 · Shift 2 · Single correct option

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