Q135 / 200 · NEET 2021 · Physics

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 :

Figure

  • A
    20 cm from the lens, it would be a real image.
  • B
    30 cm from the lens, it would be a real image.
  • C
    30 cm from the plane mirror, it would be a virtual image.
  • Correct answer
    D
    20 cm from the plane mirror, it would be a virtual image.

Solution

first, for image formation from lens
u=-60 cm

f=+30 cm

⇒v=ufu+f=-60×30-60+30=60 cm

Figure

this real image formed by lens acts as virtual object for mirror

Real image from plane mirror is formed 20 cm in front of mirror, hence at 20 cm distance from lens. Now, for second refraction from lens, u=-20 cm

f=+30 cm

v=ufu+f=-20×30-20+30=-60 cm

So, final virtual image is 60 cm from lens, or 20 cm behind mirror

Figure

Hence, the correct answer is option: 'D'.

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