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JEE Main 2026 · 4 Apr · Shift 1, Physics question 39

A rod of length 10 cm lies along the principle axis of a concave mirror of focal length 10 cm as shown in figure. The length of the image is ________ cm.

A concave mirror on the right; a 10 cm rod lies on the principal axis to its left, with the rod's nearer end 20 cm from the mirror (the rod extends from 20 cm to 30 cm).
Choose one option
Answer and worked solution

Official answer: B

Data. f=−10f=-10 cm, so R=20R=20 cm; the rod lies from 20 cm to 30 cm in front of the mirror.

Images of the ends.

  • The end at C (20 cm) images onto itself, at 20 cm.
  • The end at 30 cm: 1v=1f−1u=−110+130\frac1v=\frac1f-\frac1u=-\frac1{10}+\frac1{30}, so v=−15v=-15 cm.

Length 20−15=520-15=5 cm.

(B)

Topics:Spherical mirrors
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