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Physics · Grades 10–12

Thin lenses and image conventions

Use one sign convention and avoid claims that every convex lens image is real.

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Learn the idea

Thin-lens model

For a converging lens with a real object beyond the focal point, positive real distances satisfy 1/f = 1/do + 1/di.

Real image

Rays actually converge at a real image, which may be formed on a screen.

Virtual case

An object inside the focal length of a converging lens produces a virtual image in the elementary model.

Magnification

Image size and orientation are different. Specify whether a signed magnification or its magnitude is requested.

Worked examples

A thin converging lens has f = 10 cm and real object distance 30 cm. Use positive real distances. Find image distance.

1/di = 1/10 − 1/30 = 1/15.

15 cm

What distinguishes a real image in a ray model?

Real images correspond to actual ray convergence.

Rays actually converge there.

Check your understanding

  1. Can a converging lens form a virtual image?

    1. Yes, for an object inside its focal length.
    2. No, never.
    3. Only if it stops refracting light.

  2. An object height is 4 mm and magnification magnitude is 3. Find image height magnitude.

  3. Why should the problem's sign convention be stated before using a lens formula?

Answers and marking notes

Answers and marking notes
  1. Yes, for an object inside its focal length.

    Object position relative to the focus matters.

  2. 12 mm

    Image size is 3 × 4 = 12 mm.

  3. Different conventions encode real and virtual distances differently; mixing them leads to incorrect signs.

    Credit consistency rather than claiming one notation is universal.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.