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Exam level6 past questions

Lenses and mirrors

Explore the image formation, ray diagrams, lens and mirror formulas, and the behavior of light with concave/convex lenses and mirrors, including combinations and virtual objects.

Why this shows up in the exam

You need to analyze and predict image positions and properties in NEET questions involving optical instruments and setups.

How NEET tests this

Direct recall · 3 QsNumerical · 3 QsMulti-concept

Learn the idea

Lenses and mirrors form images by refraction or reflection according to the lens‑mirror formula and ray‑diagram rules, using object distance, image distance and focal length with sign convention.

🧠 Memory hook: F‑R‑V: Focus rule, Ray rule, Virtual‑real sign

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • For a converging lens, a ray parallel to the principal axis passes through the second principal focus after refraction.
  • For a diverging lens, a ray parallel to the principal axis appears to diverge from the first principal focus.
  • A plane mirror reflects a ray such that the object distance equals the image distance measured from the mirror surface.
  • The lens‑mirror formula relates object distance, image distance and focal length with appropriate signs.

How to approach it

  1. 1Identify the type of optical element (convex lens, concave lens or plane mirror) and the nature of the ray given.
  2. 2Apply the appropriate ray‑diagram rule to locate the intermediate image and note its distance and sign.
  3. 3Use the lens‑mirror formula or the mirror property to find the final image position, checking real/virtual nature.

Worked example — watch it click

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) 20 cm from the plane mirror, it would be a virtual image. (b) 20 cm from the lens, it would be a real image. (c) 30 cm from the lens, it would be a real image. (d) 30 cm from the plane mirror, it would be a virtual image.

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

The concept behind this problem

The example forces you to treat the image formed by the convex lens as a virtual object for the return passage, testing understanding of sign convention and the effect of a plane mirror on image distance.

Step by step

  1. 1Lens equation: 1/v - 1/(-60) = 1/30 gives v = 60 cm.
  2. 2Image forms 20 cm beyond mirror.
  3. 3Mirror reflects it back; object at 20 cm from lens gives image at 1/v - 1/(-20) = 1/30, so v = 60/7 ≈ 8.6 cm, but the first image acts as object for return path giving final real image at 20 cm from lens.

Watch out

Examiners often give the distance from the lens instead of from the mirror, leading to a wrong answer if the mirror’s role is ignored.

Common slip-ups that cost marks

  • •Confusing the sign of object distance when the first image acts as a virtual object for the second pass.
  • •Treating the plane mirror as changing the focal length instead of merely reversing the ray path.

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 6NEET 2021

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) 20 cm from the plane mirror, it would be a virtual image. (b) 20 cm from the lens, it would be a real image. (c) 30 cm from the lens, it would be a real image. (d) 30 cm from the plane mirror, it would be a virtual image.

Push further

More challenging

2 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 2

A biconvex lens has a power of 5 dioptres. If the lens is made of a material with refractive index 1.5 and both surfaces have the same radius of curvature, what is the magnitude of the radius of curvature?

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