NEB Class 11 • Physics • Focused Revision

Refraction through Prisms Study Guide: NEB 11 Physics

Study a prism as two linked refractions. Draw both normals, rebuild deviation and minimum-deviation relations, practise thin prisms and evaluate measured curves.

Seven-session prism-geometry plan
Deviation, minimum and thin-prism drills
Experiment, error log and timed checkpoint

A prism changes direction at two plane boundaries.

Learning target

What successful prism study looks like

The CDC curriculum includes minimum deviation, refractive index and small-angle prism deviation. Use the CDC Physics Grade 11 page for current Nepal material.

Mastery means reconstructing relations from a labelled diagram, not memorising an unlabelled equation. Use the prism concept guide and OpenStax prism overview when a concept gap appears.

Two-surface map

Keep geometry and physics separate

Geometry

A=r₁+r₂ and δ=i+e−A.

Physics

Snell’s law applies independently at each face.

Symmetry

At minimum deviation i=e and r₁=r₂.

Approximation

Thin prism uses small angles to obtain δ≈(n−1)A.

Seven sessions

Prism learning sequence

Session	Focus	Evidence
1	Prism diagram and normals	Ten blank labels
2	Deviation derivation	Rebuild unaided
3	Snell law at both faces	Five full ray calculations
4	Minimum deviation	Six direct/reverse problems
5	Thin prism and dispersion	Eight ratio and explanation tasks
6	δ–i experiment	Graph and uncertainty analysis
7	Timed mixed test	80% without angle-label errors

Use 55-minute blocks: ten minutes drawing, ten minutes derivation correction, thirty minutes problems and five minutes error logging.

Derivation ladder

Rebuild the core prism equations

Draw both normals and label every angle.
Use triangle geometry to show A=r₁+r₂.
Write first-face deviation i−r₁.
Write second-face deviation e−r₂.
Add to obtain δ=i+e−A.
At minimum set i=e and r₁=r₂=A/2.
Use Snell to obtain n=sin[(A+δ_m)/2]/sin(A/2).

State the prism is in air for the displayed n relation. In another outside medium, use relative refractive index.

Calculation drills

Direct, reverse and comparison practice

Find deviation

A=60°, i=50°, e=45°. δ=35°.

Find index

A=60°, δ_m=38°. n=sin49°/sin30°≈1.51.

Find δ_m

n=1.60, A=50°. sin[(A+δ_m)/2]=1.60sin25°≈0.676. The angle≈42.5°, so δ_m≈35.0°.

Thin-prism ratio

At fixed n, doubling small A doubles approximate deviation. At fixed A, deviation ratio is (n₁−1)/(n₂−1).

Graph literacy

Read the deviation-incidence curve

Deviation decreases with incidence, reaches a broad minimum and rises. A horizontal line slightly above the minimum may cross the curve twice, corresponding to reversible paths with incidence and emergence interchanged.

The minimum location supplies i≈e. If the experimental curve is asymmetric, inspect prism movement, normal construction, protractor zero and ray alignment. Do not select one visually lowest point without considering measurement resolution.

Transfer check: at the minimum, δ_m=2i−A and r₁=r₂=A/2 must agree with the labelled diagram.

Practical evidence

Plan a minimum-deviation measurement

Keep the prism fixed, vary the incident direction, trace incident and emergent rays and calculate δ from each pair. Take coarse readings first, then smaller angle steps near the minimum. Plot δ against i and estimate δ_m with uncertainty.

Error	Code	Correction
One normal reused	geometry	Draw normal at each face
Prism moved	alignment	Retrace before continuing
Wide ray	position	Mark the central path
Few points near minimum	sampling	Use smaller incidence steps
Eye aligned with beam	safety	Use screen/pins and approved source

Use PhET Bending Light for Snell predictions at each face.

Spaced correction

Repair the first wrong decision

Code face, normal, angle label, geometry, Snell, symmetry, approximation, calculator or graph. Redo after one, three and seven days. Review plane refraction if medium order or normal reference fails, and Physical Quantities for units.

Final self-test

Prism readiness checklist

Both normals and all six angles are labelled.
Geometry and Snell relations are distinguished.
Minimum-deviation symmetry is justified.
The n relation is used for a prism in air.
Thin-prism approximation is stated.
Dispersion is linked to wavelength-dependent n.
Experimental curve and uncertainty are interpreted.

Derive both geometry relations.
Solve two minimum-deviation problems.
Derive thin-prism deviation.
Explain dispersion.
Evaluate a δ–i graph.

For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions about studying prism refraction

What should I draw first?

Draw the prism, both normals and the full incident-internal-emergent path before labelling angles.

How do I avoid mixing r₁ and r₂?

Attach each refraction angle to its own face normal and use r₁+r₂=A as a check.

Why is the minimum path symmetric?

By optical reversibility, the minimum occurs when incidence equals emergence and the internal angles are equal.

How do I know whether the thin-prism formula applies?

The prism angle and ray angles must be small enough for sinθ≈θ.

What score shows readiness?

Aim for at least 80% with no repeated normal, angle-label, symmetry or approximation error.

Where can I get NEB prism 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

Use the Class 11 Physics revision roadmap to interleave mirrors, plane refraction and prisms. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Integrated challenge

From raw angles to a material conclusion

Given A=60° and measured δ values around 39° with resolution 0.5°, estimate δ_m and use the minimum-deviation relation to calculate n. Repeat for the plausible low and high δ limits to estimate how angular uncertainty affects n.

Then compare two colours. If the shorter wavelength has a slightly larger δ_m, infer a larger refractive index for that wavelength. State that the conclusion depends on the same prism geometry, alignment and temperature conditions.

Derivation and calculation workbook

Practise the same prism in four representations

Start with a labelled ray diagram. Write the two geometric relations. Apply Snell’s law at both faces. Finally sketch the δ–i curve and mark the symmetric minimum. Each representation should locate the same physical path.

Reverse index problem

A=50° and δ_m=30°. n=sin40°/sin25°≈0.643/0.423≈1.52. Check that n is above one and within a plausible range for an ordinary transparent solid given the supplied data.

Find A from thin-prism data

A small prism with n=1.60 produces approximate deviation 4.8°. A≈δ/(n−1)=4.8/0.60=8.0°. State the small-angle approximation.

Colour ratio

For a common thin A, δ_v/δ_r≈(n_v−1)/(n_r−1). The prism angle cancels, but only under the same small-angle model.

Oral explanation set

Why are two normals necessary?
Why does a prism usually deviate light toward its base?
Why is the minimum path symmetric?
Why can one deviation above the minimum correspond to two incidence angles?
Why does violet usually deviate more than red in ordinary glass?

Answer with geometry, Snell’s law, reversibility and wavelength-dependent index. Avoid saying only “because the formula says so.”

Spaced revision schedule

On day 1 derive the geometry; on day 2 solve a full two-face path; on day 4 solve a minimum-deviation reverse problem; on day 7 analyse an experimental curve. At each review, draw from an empty page and record the first missing label or assumption.

Finish with a 30-minute mixed test: two diagram labels, one full Snell calculation, two minimum-deviation questions, one thin-prism derivation, one dispersion explanation and one uncertainty item. Mark model selection and explanation as well as arithmetic.

Error log and practical judgement

Correct prism mistakes at the decision level

First error	Code	Repair
Angles from the face	normal	Draw both perpendiculars first
Used A=r₁−r₂	geometry	Rebuild the internal triangle
Set i=e away from minimum	condition	Look for δ_m or symmetry evidence
Used thin formula for 60° prism	approximation	Return to full Snell relations
Read one lowest point	graph	Fit the minimum region and report range

Safety is part of practical Physics: keep eyes out of the beam, use teacher-approved sources, mark rays on paper or screen and never align by looking along a laser path. A beautiful curve does not justify unsafe procedure.

Retest each error with an empty diagram. Explain aloud which parts come from geometry, which from Snell’s law and which from a stated approximation. This separation prevents a familiar equation from being applied to the wrong prism condition.

Weekly transfer: invent a prism problem where the path is not at minimum, another that uses δ_m, and a third that permits the thin approximation. Write the evidence that selects each method before solving. Exchange the questions and mark diagram labels, conditions, approximations and physical checks separately. After three days, solve them in shuffled order without method headings and explain why each discarded formula is unsuitable.

Keep the marked attempts so repeated geometry and condition errors can be distinguished from isolated arithmetic slips during the next spaced review, then discussed carefully in a detailed oral explanation session.

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