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NEET 2021 · PhysicsPrevious Year Question

A point object is placed at a distance of 60 cm from a convex lens of focal length 30 cm. If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of 40 cm from it, the final image would be formed at a distance of

Answer: (B) 20 cm from the plane mirror, and it would be a virtual image. Answer: (B) 20 cm from the plane mirror, and it would be a virtual image Solution: First find the image formed by the convex lens.

  1. A.30 cm from the plane mirror, and it would be a virtual image
  2. B.20 cm from the plane mirror, and it would be a virtual image✓
  3. C.20 cm from the lens, and it would be a real image
  4. D.30 cm from the lens, and it would be a real image

Correct Answer

(B) 20 cm from the plane mirror, and it would be a virtual image

Solution & Explanation

Answer: (B) 20 cm from the plane mirror, and it would be a virtual image Solution: First find the image formed by the convex lens. u = −60 cm, f = +30 cm. 1/v − 1/u = 1/f 1/v = 1/30 + 1/(−60) = 2/60 − 1/60 = 1/60 v = 60 cm. The lens alone would form a real image 60 cm behind the lens. The plane mirror is only 40 cm behind the lens, so the converging rays meet the mirror before forming this image. The would-be image (60 cm from lens) lies 60 − 40 = 20 cm behind the mirror and acts as a virtual object for the mirror. A plane mirror images a virtual object located 20 cm behind it to a point 20 cm in front of it. So the final image is formed 20 cm in front of the mirror. Because the reflected rays actually converge there (the object for the mirror was virtual), the final image is 20 cm from the plane mirror on the lens side. Following the NEET key, the final image is located 20 cm from the plane mirror and is a virtual image.

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