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NEET PHYSICSRAY OPTICS AND OPTICAL INSTRUMENTSMedium

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:

A

30 cm from the plane mirror, it would be a virtual image.

B

20 cm from the plane mirror, it would be a virtual image.

C

20 cm from the lens, it would be a real image.

D

30 cm from the lens, it would be a real image.

Step-by-Step Solution

  1. Image formation by Lens: Using the lens formula 1v−1u=1f\frac{1}{v} - \frac{1}{u} = \frac{1}{f} with u=−60 cmu = -60\text{ cm} and f=+30 cmf = +30\text{ cm}: 1v−1−60=130  ⟹  1v=130−160=160\frac{1}{v} - \frac{1}{-60} = \frac{1}{30} \implies \frac{1}{v} = \frac{1}{30} - \frac{1}{60} = \frac{1}{60} v=+60 cmv = +60\text{ cm} In the absence of the mirror, a real image (I1I_1) would form 60 cm to the right of the lens.
  2. Interaction with Plane Mirror: The mirror is placed 40 cm to the right of the lens. The rays converging towards I1I_1 (at 60 cm) are intercepted by the mirror.
  • The image I1I_1 acts as a virtual object for the plane mirror.
  • Distance of virtual object from mirror (umu_m) = 60 cm(I1)−40 cm(Mirror)=20 cm60\text{ cm} (I_1) - 40\text{ cm} (Mirror) = 20\text{ cm} (behind the mirror).
  1. Final Image Formation: A plane mirror forms an image of a virtual object at the same distance in front of it.
  • Image distance from mirror (vmv_m) = 20 cm (in front of the mirror).
  • Since the reflected rays actually converge at this point, the final image (I2I_2) is Real.
  1. Position relative to Lens: The image is formed 20 cm in front of the mirror (towards the lens). Since the mirror is 40 cm from the lens, the distance of the image from the lens is 40 cm−20 cm=20 cm40\text{ cm} - 20\text{ cm} = 20\text{ cm}.
  2. Conclusion: The final image is formed 20 cm from the lens (or 20 cm from the mirror) and is a real image. Option 3 correctly describes both position and nature.

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from RAY OPTICS AND OPTICAL INSTRUMENTS. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSRAY OPTICS AND OPTICAL INSTRUMENTSobjectplaceddistanceconvexlength

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