Explain white-light dispersion by a glass prism
Follow different wavelengths through two nonparallel faces and explain the separated spectrum.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Follow the selected Light section; ray diagrams use an ideal thin-lens or plane-boundary model. Written work is self-checked. Human teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Trace entry and exit refraction at transparent boundaries.
- Distinguish a prism from a parallel-sided block.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can white light form a prism spectrum?
What you will learn
- Describe prism dispersion of white light.
- Explain why the emergent colours have different deviations.
Different wavelengths refract differently
White light contains a range of visible frequencies and wavelengths. A glass prism refracts the light on entering and leaving its nonparallel faces. For ordinary visible-light dispersion in glass, the refractive index is generally greater for violet than for red, so the components follow different paths and leave with different overall deviations.
In the usual prism spectrum, red is deviated least and violet most; intermediate colours appear between them. The prism separates existing components rather than manufacturing colours from one frequency. A narrow white-light input and suitable projection arrangement make the spread easier to observe than a wide overlapping beam.
Read an ordinary spectrum with the right scope
A ray sketch can show representative red, green and violet paths while the real visible spectrum is continuous. The traditional seven names label regions of that continuous range, not seven isolated frequencies with empty spaces between them. The supplied original prism geometry is a teaching model, not a performed spectrum measurement.
A single-frequency input can still be refracted and deviated by the prism, but it cannot be dispersed into the full range already present in white light. A parallel-sided block can give wavelength-dependent lateral shifts while restoring parallel emergence in the matching outside medium; the prism's nonparallel faces make the separated net directions clear.
Pause and explain
Which visible colour has the greatest usual deviation in a glass-prism spectrum?
Worked example
A narrow white beam passes through an ordinary glass prism. Compare its red and violet emergent directions.
Show the worked solution
- Both components refract at entry and exit.
- The glass's index differs for the two components.
- Violet shows greater overall deviation than red in the usual model.
Answer Red deviates least and violet most; the emergent spectrum separates white-light components.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how a glass prism disperses a narrow white beam.
Give me a hint
Follow both boundaries and compare the components.
Compare my reasoning
- White light contains different visible components.
- Their refraction differs at the prism faces.
- They emerge in different directions as a spectrum.
Wavelength-dependent refraction at nonparallel faces separates the components.
Look for these in your work
- Use both boundaries.
- Keep the components' origin in the input.
Compare red and violet deviation in the ordinary prism model.
Give me a hint
Use the usual visible-light dispersion ordering.
Compare my reasoning
- Red and violet both undergo refraction.
- Violet has the greater usual glass index.
- Violet is deviated more and red less.
Violet most; red least.
Look for these in your work
- State the correct order.
- Link it to differing refraction.
A diagram shows only red, green and violet rays. Does this prove the spectrum contains exactly three frequencies?
Give me a hint
Representative drawing paths are not a full frequency inventory.
Compare my reasoning
- The three paths illustrate different deviations.
- White light has a continuous visible range.
- The drawing omits intermediate components for clarity.
No; representative rays illustrate a continuous spectrum.
Look for these in your work
- Distinguish model from inventory.
- Avoid inventing gaps between named colours.
Common mistakes
- The prism separates components rather than creating frequencies.
- Violet has greater usual prism deviation than red.
- Seven named colours describe a continuous visible spectrum.
What can you explain now?
Does a prism turn one single frequency into all seven white-light colours?
Compare with the explanation
No.
Dispersion separates the frequencies present in the input.
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Make it stick
Explain prism dispersion, the red/violet ordering and the meaning of representative spectrum rays.
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