NEB Class 11 • Physics • Focused Revision

Dispersion Study Guide: NEB 11 Physics

Build one causal chain from wavelength to refractive index, deviation, spectrum and application. Practise explanations and calculations without confusing dispersion with diffraction or scattering.

Six-session cause-to-application plan
Prism calculations and spectrum reasoning
Experiment analysis, error log and timed test

Dispersion is refraction that depends on wavelength.

Revision boundary

Use current scope and authoritative explanations

Check the CDC secondary curriculum and CDC Physics Grade 11 page. Study the paired Dispersion concept guide, then compare with OpenStax dispersion, rainbows and prisms.

Prerequisites are Snell’s law, prism geometry, minimum deviation, wavelength, frequency and the small-angle approximation. If those are weak, use the Refraction through Prisms Study Guide first.

Causal chain

Memorise relationships, not colour slogans

Wavelength

Incident white light contains a continuous distribution of spectral components.

Index n(λ)

Material response gives different refractive indices at different wavelengths.

Deviation δ(λ)

Snell’s law and prism geometry produce wavelength-dependent paths.

Observation

A spread spectrum, rainbow, chromatic fringe or measured spectral signal.

Practise speaking the chain both forward and backward. If violet deviates more, infer a larger visible-range refractive index for violet in ordinary glass under the stated conditions.

Six sessions

A focused dispersion revision sequence

Session	Focus	Exit evidence
1	Wave language and n(λ)	Five cause-and-effect sentences
2	Prism path and colour order	Labelled spectrum diagram
3	Thin-prism and minimum-deviation maths	Six mixed calculations
4	Rainbow and chromatic aberration	Two application explanations
5	Practical design and uncertainty	Method critique
6	Timed integrated test	80% without mechanism confusion

Each 50-minute session begins with closed-book retrieval and ends with one transfer question. Space reviews after one, three and seven days.

Calculation routine

Write the approximation beside the formula

For a thin prism in air, δ≈(n−1)A and Δδ≈(n_v−n_r)A when angles are small and expressed consistently. At minimum deviation for a general prism, n=sin[(A+δ_m)/2]/sin(A/2). Use different n values for different wavelengths.

Angular spread

A thin prism has A=6° and n_v−n_r=0.020. Δδ≈0.120°. In radians this is 0.00209. At L=1.5 m, small-angle screen separation is y≈LΔδ≈3.14 mm.

Find index difference

A 4° thin prism produces violet–red separation 0.080°. Then n_v−n_r≈Δδ/A=0.080/4=0.020, provided both angles use the same unit in the ratio.

Full minimum-deviation comparison

For A=60°, δ_m,r=38° gives n_r=sin49°/sin30°≈1.509. If δ_m,v=40°, n_v=sin50°/sin30°≈1.532. The supplied values give a difference near 0.023.

Long-answer method

Build explanations in four moves

Name the incident spectrum and wavelength components.
State that refractive index depends on wavelength.
Apply direction: in ordinary glass violet is generally refracted and deviated more than red.
Describe the observed result and the geometry or instrument.

Rainbow answer

Sunlight contains a continuous visible spectrum. On entering a droplet it is refracted and dispersed; after an internal reflection it is refracted again on exit. Different wavelengths emerge at different directions, and the observer receives each colour from droplets at appropriate angular positions. The arc is therefore an observer-centred angular pattern.

Chromatic aberration answer

A simple lens has wavelength-dependent refractive index, so its optical power and focal length vary with colour. The components may not focus at one plane, causing coloured fringes or blur. Combining suitable glasses can reduce the difference over selected wavelengths.

Practical analysis

Separate spectrum width from measurement uncertainty

A narrow slit creates a narrow image for each wavelength and makes bands easier to distinguish. A broad slit does not change the wavelengths, but overlapping images reduce spectral resolution. Dim ambient conditions improve contrast; a longer screen distance increases linear separation but may lower brightness and magnify alignment errors.

Predict red and violet sides before placing the screen.
Use a safe teacher-approved white source and never look along a laser beam.
Keep slit, prism and screen aligned.
Mark colour extremes with a stated criterion.
Measure repeat positions after realignment.
Convert linear spread to angle only with the stated geometry or small-angle approximation.

Uncertainty comes from diffuse colour boundaries, slit width, prism rotation, screen-distance measurement and choice of the colour “edge.” Report a range rather than false precision.

Error log

Distinguish nearby wavelength phenomena

Statement	Problem	Repair
A prism makes seven colours	Discrete-spectrum misconception	Say it separates a continuum already present
Violet frequency decreases in glass	Boundary error	Frequency stays fixed; speed and wavelength change
Rainbow is only TIR	Incomplete path	Include entry/exit refraction and internal reflection
All separation is dispersion	Mechanism confusion	Contrast refraction, diffraction and scattering
δ=(n−1)A exactly	Approximation hidden	State thin prism and small angles

Thirty-minute test

Readiness tasks and marking checks

Define dispersion precisely.
Draw and label white light through a prism.
Explain n(λ) and colour order.
Calculate thin-prism angular dispersion and screen separation.
Find two refractive indices from minimum deviations.
Explain the primary-rainbow path.
Explain chromatic aberration and one reduction strategy.
Evaluate a slit–prism–screen experiment.
Distinguish dispersion from diffraction and scattering.

The spectrum is described as continuous.
Material and wavelength dependence are stated.
Small-angle assumptions are visible.
Frequency is not incorrectly changed at a boundary.
Every calculation ends with a physical comparison.
Safety and uncertainty are specific.

Use PhET Bending Light only after predicting the effect. For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions about studying dispersion

What is the core sentence to remember?

Different wavelengths experience different refractive indices and therefore different refraction or deviation in a given optical system.

Should I memorise seven colours?

Know the common order, but state that the visible spectrum is continuous rather than seven discrete wavelengths.

How do I avoid using the wrong prism formula?

Look for thin/small-angle evidence before using δ≈(n−1)A; otherwise use full prism geometry and Snell’s law.

How should I revise rainbow formation?

Draw entry refraction and dispersion, internal reflection, exit refraction and the observer’s angular selection.

What makes an experiment answer strong?

Include alignment, slit width, repeat measurements, boundary-position uncertainty, safe viewing and the angular conversion model.

Where can I get dispersion tuition?

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

References and next steps

Sources and related study guides

Review the preceding Lenses Practice Set to keep optical models connected. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Application workbook

Four transfer problems beyond a memorised spectrum

1. Screen-distance design

A prism gives angular red–violet separation 0.12°. Convert to radians: 0.12π/180≈0.00209. To obtain 5.0 mm separation under a small-angle screen model, L≈y/Δδ=0.005/0.00209≈2.39 m. A real setup must also consider brightness, available room and band width.

2. Rank two materials

Material A has n_v−n_r=0.018 and material B has 0.030 for the same chosen wavelengths. Equal thin prism angles give larger angular dispersion for B. This does not by itself state which material has greater mean deviation; compare n_mean−1 separately.

3. Diagnose a broad spectrum

A student widens the slit and sees brighter but less distinct bands. The source spectrum is not less dispersed; each wavelength’s slit image is wider and neighbouring images overlap. Repair by narrowing the slit, reducing ambient light, refocusing and checking prism alignment.

4. Achromatic reasoning

Two lens materials can be combined so their colour-dependent powers partly cancel while useful mean power remains. A correct explanation says “reduce selected chromatic differences,” not “remove all dispersion.” The design depends on glass data, curvatures and wavelengths.

Claim–evidence–limit writing

Practise three-sentence answers. Claim: violet deviates more than red in ordinary visible-light glass. Evidence: n_v>n_r, so Snell refraction and prism deviation are larger. Limit: material behaviour can differ outside the stated wavelength range. Use the same pattern for rainbows, chromatic aberration and slit width.

Mini oral examination

Why does frequency remain fixed while wavelength changes?
Why is a prism spectrum continuous?
Which parts of a primary-rainbow path are refraction and which reflection?
When is Δδ≈(n_v−n_r)A justified?
Why can two instruments with equal angular dispersion have different resolving ability?

Answer each without notes, then check mechanism words against the concept guide. Retest after three days with the order shuffled. A response that gives only the colour order earns less evidence than one that connects source spectrum, n(λ), geometry and observation.

Weekly transfer: draw a prism spectrum, rainbow droplet and chromatic lens focus on one page. Under each, write the common n(λ) idea and the different geometry. Then contrast each with diffraction and scattering so a colour observation never selects a mechanism by itself.

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