Lenses: NEB Class 11 Physics Guide

Learning boundary

What the NEB lens topic asks you to connect

The CDC secondary curriculum includes focal length of a convex lens and geometrical-optics relations. Check the CDC Physics Grade 11 page for current Nepal materials. This guide develops the same ideas with the institutional treatments in OpenStax image formation by lenses and OpenStax high-school lenses.

Prerequisites are refraction, a normal to a surface, refractive index, similar triangles and reciprocal algebra. Review Refraction at Plane Surfaces before deriving lens behaviour.

Optical language

Name each point before drawing rays

A lens thicker at the centre than the edge is usually converging in air; one thinner at the centre is usually diverging. The surrounding medium matters: convergence is not determined by shape alone in every possible medium.

Construction method

Use principal rays as evidence

1. Draw the axis, lens line, O and equal focal marks on both sides.
2. Place the object relative to F and 2F.
3. From its tip draw a ray parallel to the axis. A converging lens sends it through the far focus; a diverging lens sends it as if from the near focus.
4. Draw a ray through O with negligible deflection in the thin-lens model.
5. Use a focus-to-parallel ray as a check.
6. Mark the image where real rays meet or where dashed backward extensions appear to meet.

Do not bend the centre ray at O merely to make it intersect. Do not extend real rays backwards with solid lines. A partial aperture still admits rays from every object point, so it makes the full image dimmer rather than cutting away exactly half of it.

Image prediction

Map object region to image nature

Reconstruct the table from ray rules rather than memorising six isolated lines. The screen test is decisive: real rays can form a sharp image on a screen; a virtual image cannot be caught directly without another optical element.

Quantitative model

Make the equation agree with the diagram

In the OpenStax real-is-positive convention, 1/f=1/d_o+1/d_i and m=h_i/h_o=−d_i/d_o. Real image distance is positive, virtual image distance negative, converging f positive and diverging f negative. Your school may use a Cartesian convention; state it and do not mix signs from different systems.

Focal strength

Convert focal length to lens power

P=1/f when f is in metres, so power is measured in dioptres (D). A +2.5 D lens has f=+0.40 m. A −4.0 D lens has f=−0.25 m. Convert centimetres to metres before taking the reciprocal; a 20 cm converging lens is +5 D, not 0.05 D.

For thin lenses in contact in the paraxial approximation, powers add: P_eq=P₁+P₂. A +5 D and −2 D pair gives +3 D, or f≈0.333 m. State that spacing, thickness and aberrations limit this simple model.

Laboratory evidence

Measure focal length safely and critically

Use an illuminated distant object—not the Sun—and form a sharp image on a screen with a convex lens. Measure from O to the screen for an approximate focal length. Repeat after refocusing and quote the spread. A finite object distance, lens thickness, parallax, screen judgement and imperfect alignment cause uncertainty.

For a u–v method, take several object distances greater than f, focus the screen, and record d_o and d_i. Calculate f for each trial or plot 1/d_i against 1/d_o. The equation predicts intercept 1/f and gradient −1 under the chosen axes. Use PhET Geometric Optics after predicting each movement.

Design reasoning

Choose a lens from the required image

A camera needs a real image on a sensor; a magnifier uses a convex lens with the object inside F; a diverging corrective lens can reduce effective convergence for myopia. A compound microscope and telescope use more than one lens, so analyse one image stage at a time. Do not recommend a spectacle power from a classroom calculation—eye care requires a qualified professional.

Common mistakes and repair

• Angles or distances measured from a surface rather than O: redraw the axis and reference.
• Using centimetres in P=1/f: convert to metres first.
• Calling every convex-lens image real: check whether the object is outside F.
• Giving only a number: interpret image side, orientation and size.
• Mixing mirror and lens ray rules: label reflection or refraction at the device.
• Using a formula without a sketch: make a region prediction before algebra.

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References and next steps

Sources and related study guides

• CDC Nepal: Physics Grade 11
• CDC Nepal: Secondary Curriculum
• OpenStax: Image Formation by Lenses
• OpenStax: Thin Lenses
• OpenStax: High-school Lenses
• PhET: Geometric Optics

Continue with the Lenses Study Guide and use the Physical Quantities guide for unit and uncertainty review. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Integrated transfer

Trace an image through two lens stages

Compound optical instruments are best handled one stage at a time. Suppose a first convex lens forms a real inverted intermediate image. That image becomes the object for a second lens, and its object distance is determined by the separation between the second lens and the intermediate image—not automatically by the original object distance. Draw the intermediate image before writing the second equation.

This stage method also reveals model limits. If the intermediate image would lie beyond the second lens, it may act as a virtual object and the chosen sign convention must handle that case explicitly. Lens separation means powers cannot simply be added as though the elements were in contact.