NEB Class 11 • Physics • Focused Revision

Refraction at Plane Surfaces Study Guide: NEB 11 Physics

Turn every refraction question into a boundary diagram, direction prediction and tested relation. Practise Snell’s law, apparent depth, slabs and critical angle with spaced retrieval.

Seven-session boundary-diagram plan
Snell, depth, slab and critical-angle drills
Experiment, error log and timed checkpoint

Angles are always measured from the normal.

Learning target

What successful refraction study looks like

The CDC curriculum covers laws of refraction and lateral shift. Use the CDC Physics Grade 11 page for current Nepal material and OpenStax Refraction for institutional reference.

Mastery means predicting direction before trigonometry, measuring angles from the normal, knowing when an approximation applies and connecting diagrams with data. Rebuild theory in the concept guide.

Boundary map

Ask four questions at every surface

Which media?

Label n₁ and n₂ in travel order.

Which normal?

Draw perpendicular at the crossing point.

Which direction?

Predict toward or away from normal.

Which model?

Snell, apparent depth, slab geometry or critical angle.

Seven sessions

Plane-refraction study sequence

Session	Focus	Evidence
1	n=c/v, normal and bending	Twelve prediction cards
2	Snell’s law	Eight direct/reverse problems
3	Apparent depth	Five diagrams and calculations
4	Parallel slab shift	Four geometry problems
5	Critical angle and TIR	Six condition checks
6	sin i–sin r experiment	Graph and uncertainty analysis
7	Timed mixed set	80% without angle-reference errors

Use 50-minute blocks: eight minutes retrieval, twelve minutes targeted review, twenty-five minutes problems and five minutes error coding.

Six-step routine

Predict before pressing inverse sine

Draw the plane boundary and normal.
Label n₁, n₂ and travel direction.
Predict whether θ₂ is smaller or larger.
Write n₁sinθ₁=n₂sinθ₂.
Solve symbolically and check sine range.
Compare the calculated angle with the prediction.

Quick ratio

From air into n=1.50 at 45°, sinθ₂=0.707/1.50=0.471, so θ₂≈28.1°. The smaller angle confirms toward-normal bending.

Calculator mode must be degrees. A result larger than the incident angle for entry into higher n is a warning.

Mixed model drills

Apparent depth, slab shift and critical angle

Apparent depth

A coin is 1.2 m below water surface, n=1.33, viewed nearly normally from air. Apparent depth≈0.902 m; apparent rise≈0.298 m.

Unknown index

Real depth 15 cm appears 10 cm from air. Relative index≈15/10=1.50 under the paraxial model.

Slab shift scaling

At fixed i and material, doubling slab thickness doubles lateral shift. At normal incidence shift is zero.

Critical angle

For glass n=1.60 to air, θ_c=sin⁻¹(1/1.60)≈38.7°. State higher-to-lower travel first.

Graph literacy

Use sin i against sin r

For air-to-block, sin i=n sin r approximately, so a graph of sin i on the vertical axis against sin r on the horizontal axis has gradient n. Reverse axes and the gradient becomes 1/n; axis labels decide the interpretation.

A line not passing through the origin may indicate angle zero offset or block movement. Scatter reflects reading uncertainty. Use a best-fit gradient across a wide safe angle range rather than averaging rounded ratios.

Transfer check: a steeper sin i versus sin r line means a larger refractive index for the second medium when the first is air.

Practical evidence

Design a reliable block experiment

Use a narrow ray, trace the block without moving it, mark two points on each path and draw normals only after joining the lines. Measure with a protractor from the normal. Repeat several i values and include uncertainty.

Error	Code	Correction
Angle from surface	reference	Draw and use normal
Wide ray	position	Use narrow central line
Block moved	geometry	Retrace and hold position
Few angles	range	Use several safe values
Forced origin	analysis	Inspect intercept and zero error

Use PhET Bending Light for one-variable prediction–observation–explanation.

Spaced correction

Repair the first wrong decision

Code medium order, normal, direction, equation, approximation, calculator, graph or unit. Retest after one, three and seven days. Link to curved-mirror ray practice for construction habits and Physical Quantities for dimensions.

Final test

Readiness checklist

Angles are measured from the normal.
Medium order and bending prediction are explicit.
Snell’s law sine values are physically possible.
Apparent-depth approximation is stated.
Slab emergence and displacement are distinguished.
Both TIR conditions are named.
Graph slope follows labelled axes.

Solve three Snell-law questions.
Complete apparent-depth and slab items.
Find two critical angles.
Interpret a sin graph.
Evaluate an experiment.

For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions about studying plane refraction

What should I draw first?

Draw the boundary and normal, then label the two media in the direction of travel.

How do I predict bending?

Entering a higher index bends toward the normal; entering a lower index bends away.

When can I use real depth over apparent depth?

For near-normal viewing through a plane interface under the paraxial approximation.

How do I remember TIR conditions?

Light must travel from higher to lower index and incidence must exceed the critical angle.

What score shows readiness?

Aim for at least 80% with no repeated normal, medium-order or approximation error.

Where can I get refraction tuition?

For current online or physical options, call 9846662070 and confirm timetable, class mode, teacher availability and fees.

References and next steps

Sources and related study guides

Next study Refraction through Prisms. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Transfer challenge

Explain a pencil in water from three viewpoints

Draw rays from one point on the submerged pencil to an observer. Explain with Snell’s law why they bend away from the normal, with apparent-depth geometry why the point seems raised, and with wave language why speed and wavelength change while frequency stays fixed.

Now move the observer sideways. The simple near-normal depth ratio becomes less exact, so use explicit ray tracing. State what changed: the object did not move, but the virtual image inferred from different ray bundles can shift.

Worked revision workbook

Five problems that change the model, not just the numbers

1. Normal incidence

A ray enters glass along the normal. Snell’s law gives both angles zero. Speed and wavelength decrease, but direction does not change. This separates refraction as a speed change from visible bending.

2. Impossible transmission check

From glass n=1.50 to air, an assumed incidence of 50° gives required sinθ₂=1.50sin50°≈1.15. Since sine cannot exceed one, no refracted ray exists in the simple model; incidence exceeds the critical angle and total internal reflection occurs.

3. Apparent depth with observer medium

An object in glass n=1.50 is viewed nearly normally from water n=1.33 at real depth 12 cm. Apparent depth≈(1.33/1.50)×12≈10.6 cm. Do not automatically divide by 1.50 when the observer is not in air.

4. Slab displacement trend

For a fixed slab and material, displacement is zero at i=0 and increases as incidence grows within the permitted range. The emergent ray remains parallel when outer media match and faces are parallel.

5. Graph-axis reversal

If sin r is plotted vertically against sin i horizontally for air-to-glass, gradient is about 1/n. Write the equation in y=mx form before reading any graph.

Turn each solution into a flash card with front: boundary diagram only; back: direction, equation, approximation and one limiting check. Shuffle plane refraction with curved-mirror cards so the normal and ray direction must be identified rather than assumed from chapter order.

Timed retrieval set

Twenty-five minute mixed checkpoint

Draw air-to-glass and glass-to-air rays at 40° incidence.
Find the refracted angle for n=1.50 from air.
Find an unknown index from two measured angles.
Calculate apparent depth and apparent rise.
Explain why a parallel slab produces displacement but no net angular deviation.
Find the critical angle for n=1.60 to air.
Decide whether incidence 45° gives transmission or TIR.
Interpret the gradient of sin i against sin r.

Mark one point each for boundary diagram, normal reference, direction prediction, correct model, algebra, unit or angle, and limiting check. A correct final angle without the normal or medium order is incomplete evidence.

After marking, choose the earliest failed decision and write a new question that changes the media rather than only the numerical angle. Retest after forty-eight hours.

Weekly transfer: photograph or sketch a straw, coin or tile edge viewed through water. Predict the shift, identify the ray bundle reaching the observer and explain why the near-normal formula may or may not apply. Do not use the observation as a precision measurement without controlling depth, angle and camera position. Repeat from a more oblique viewpoint and explain which assumption changed and why a full Snell construction becomes preferable.

Always finish by checking whether the calculated ray bends in the predicted direction.

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