NEB Class 11 • Physics • Focused Revision

Lenses Study Guide: NEB 11 Physics

Turn lens theory into a repeatable study system: retrieve the ray rules, predict image nature, calculate with one sign convention, and explain experimental evidence.

Seven-session revision sequence
Diagram–equation–language checks
Timed tasks, error log and practical analysis

A diagram should predict the sign and size of the image.

Revision boundary

Study the curriculum as connected evidence

Use the CDC secondary curriculum and CDC Physics Grade 11 page to keep scope current. Read the paired Lenses concept guide for explanations and OpenStax lens image formation for an institutional comparison.

A complete answer links four things: object region, ray diagram, signed equation and image language. If one disagrees, locate the first decision that failed instead of changing the final sign by guesswork.

Knowledge map

Organise lenses into four layers

Geometry

Axis, O, F, 2F, object and image positions.

Ray rules

Parallel–focus, focus–parallel and optical-centre rays.

Algebra

Thin-lens equation, magnification and reciprocal power.

Evidence

Screen test, focal-length experiment, uncertainty and applications.

Make one A4 page with these layers in columns. Each new question should move horizontally across the page, not remain as a disconnected formula.

Seven sessions

A spaced lens revision plan

Session	Focus	Exit evidence
1	Vocabulary and lens types	Label two blank diagrams
2	Convex principal rays	All five object regions
3	Concave lens and virtual extensions	Four varied object positions
4	Formula and sign convention	Eight mixed calculations
5	Power and lens combinations	Six unit-safe problems
6	u–v practical and graph	Method plus uncertainty critique
7	Timed integrated set	80% with no repeated model error

Use 55-minute sessions: ten minutes closed-book retrieval, ten minutes correction with coloured annotations, thirty minutes mixed questions and five minutes updating an error log.

Drawing practice

Make every line carry meaning

Use a ruler for the principal axis and mark equal focal distances.
Place the object before choosing rays.
Start rays from the same object point.
Draw arrows in the direction light travels.
Use dashed backward extensions only for a virtual image.
Label the final image as real/virtual, upright/inverted and enlarged/same/diminished.

Repeat each diagram from an empty page after one day, three days and seven days. On the second attempt, change the object region. On the third, cover the lens name and infer converging or diverging behaviour from the rays.

Limit reasoning

As a convex-lens object approaches F from outside, the real image moves farther away and grows. At F the ideal emerging rays are parallel. Moving inside F changes the image to virtual and upright. This continuous story is stronger than memorising separate table rows.

Calculation routine

Predict, define, substitute, interpret

Sketch the object region and predict image nature.
State the sign convention.
Write knowns with units and signs.
Rearrange before substitution.
Calculate image distance, magnification or power.
Translate signs and magnitude into a sentence.
Check against the sketch and a limiting case.

Object between F and 2F

Let a convex lens have f=20 cm and d_o=30 cm. Then 1/d_i=1/20−1/30=1/60, so d_i=60 cm and m=−2. The image lies beyond 2F, real, inverted and enlarged.

Unknown focal length

A real object at 40 cm forms a real image at 24 cm. Then 1/f=1/40+1/24=1/15, giving f=15 cm and P=1/0.15≈+6.67 D. The positive power agrees with convergence.

Lens combination

Two thin lenses in contact have powers +4 D and −1.5 D. Their equivalent power is +2.5 D, so f=+0.40 m. This assumes thin elements, negligible separation and paraxial rays.

Practical study

Learn the method by defending every measurement

For a convex-lens u–v experiment, align the object, lens centre and screen at the same height. Measure distances from the optical centre, not the stand edge. Focus by approaching the sharp position from both directions and record a reasonable uncertainty.

Take several object distances outside F. Calculate f for each trial or plot 1/d_i vertically against 1/d_o horizontally; the ideal straight-line gradient is −1 and vertical intercept 1/f. Scatter can arise from lens thickness, parallax, imperfect alignment, screen-focus judgement and aberration.

Use PhET Geometric Optics in a predict–observe–explain loop. A simulation supplies an ideal model, not experimental uncertainty.

Error correction

Code the first wrong decision

Error	Code	Repair task
Wrong focus used	ray	Redraw both foci before rays
Mixed conventions	sign	Write a sign table from memory
Power off by 100	unit	Convert five cm values to metres
Correct number, wrong image words	interpret	Explain sign and \|m\| aloud
Graph gradient reversed	axes	Write y=mx+c first

Redo the corrected question after 24 hours without looking at the solution. If the same code repeats, design a contrast question—for example, swap a convex lens outside F for one inside F.

Final checkpoint

Forty-minute readiness test

Label lens vocabulary and distinguish convergence from divergence.
Draw three convex cases and one concave case.
Solve one real-image and one virtual-image equation.
Convert focal length and power in both directions.
Analyse a two-lens-in-contact problem.
Explain a u–v graph and two uncertainties.
Select a lens for a stated image requirement.
Explain why half a lens still forms a full but dimmer image.

Exactly one sign convention is used.
Object region is stated before equations.
Virtual lines are dashed.
Power uses metres.
Every numerical result receives a physical interpretation.
Experimental limitations are specific, not “human error.”

For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions about studying lenses

Should I memorise the image table first?

Reconstruct it from principal rays, then use the table for fast retrieval. Understanding the transition at F makes memory more reliable.

How do I avoid sign mistakes?

Predict the image, state one convention, attach signs to every known and compare the calculated region with the sketch.

How often should I redraw diagrams?

Use spaced attempts after one, three and seven days, changing the object region each time.

What should I write after finding magnification?

State orientation from its sign and relative size from its magnitude, then check whether that matches the image case.

How do I study the practical?

Know the alignment, measurement reference, focusing method, repeat strategy, graph relation and specific uncertainty sources.

Where can I get lens tuition?

Call 9846662070 for current online or physical NEB tuition schedules, teacher availability and fees.

References and next steps

Sources and related study guides

Test the plan with the Lenses Practice Set, then revisit plane-surface refraction if Snell-law reasoning is weak. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Revision workbook

Five transfer drills for durable understanding

1. Missing half of a lens

Predict before observing: every object point sends many rays through different parts of the aperture. Covering half removes some rays from every image point, so the complete image remains but is less bright. Image location and size do not change in the ideal model.

2. Move the screen, not the object

A real image is sharp only at the conjugate image plane. Moving the screen alone causes blur; it does not move the actual geometric image. To refocus, change object distance, lens position, focal length or the sensor plane.

3. Limit at infinity

For an object very far away, 1/d_o approaches zero, so d_i approaches f for a converging lens. This explains the distant-object focal-length estimate and provides a limiting check on calculations.

4. Compare mirror and lens signs

Both systems can use similar conjugate equations, but their physical sides and common conventions differ. Write the propagation direction and definitions of positive distance before borrowing any formula. A physical image cannot change merely because a textbook labels axes differently.

5. Design from image requirements

For a projector, require a real enlarged image on a screen and therefore place the object between F and 2F of a converging lens. For a handheld magnifier, require an upright enlarged virtual image and place the object inside F. Begin with the desired image, not the device name.

Oral defence checklist

For each drill, explain which rays meet, whether a screen can capture the image, what the equation predicts and which assumption is being used. Record a 90-second explanation and listen for vague phrases such as “the lens flips it” or “power becomes bigger.” Replace them with geometry, signs and units.

Finish by writing two contrast questions that share the same focal length but place objects on opposite sides of F. Solve them after 48 hours in shuffled order; the goal is to select the model without chapter-order clues.

Continue planning