Distinguish monochromatic light from a white-light spectrum
Define a single frequency and separate refraction from dispersion into different components.
Exam scope and assessment notes
Extended only (Supplement) · 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
- Distinguish source frequency from material wavelength.
- Explain prism dispersion of white light.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What defines monochromatic visible light in the syllabus model?
What you will learn
- Recall that monochromatic visible light has a single frequency.
- Explain its prism behaviour without inventing new colours.
Use the frequency definition
Monochromatic visible light is described as light of a single frequency. In a given uniform region that frequency corresponds to one wavelength through v = fλ. Real sources can have a narrow frequency spread rather than a mathematically perfect single value; the syllabus uses the ideal single-frequency definition.
White light includes a range of visible frequencies, so it is not monochromatic even if the combined beam looks white. A colour name alone does not prove a source has exactly one frequency: different narrow or broad distributions can give similar perceived colours. Use the specified frequency information rather than the colour label alone.
Separate deviation from component separation
A monochromatic beam can refract at both prism faces and emerge with a net deviation. Because the ideal input contains only one frequency, the prism does not split it into the full white-light spectrum. Refraction changes direction and wavelength in a material but does not create additional transmitted frequencies at a stationary ordinary boundary.
For a supplied frequency 6.0 × 10¹⁴ Hz, vacuum speed 3.0 × 10⁸ m/s gives wavelength 500 nm. In a supplied material speed 2.0 × 10⁸ m/s the wavelength is about 333 nm while frequency remains 6.0 × 10¹⁴ Hz. The changed material wavelength does not make this ideal transmitted light a mixture of frequencies.
Pause and explain
A single-frequency beam is refracted by a stationary prism. What follows?
Worked example
A single-frequency visible beam enters an ordinary stationary glass prism. Does it form all seven white-light colours?
Show the worked solution
- The input contains one frequency in the ideal model.
- It refracts and may change overall direction.
- No different-frequency components are supplied for a full white-light spectrum.
Answer It can be deviated, but it does not become a full white-light spectrum.
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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.
Define monochromatic visible light.
Give me a hint
Use the frequency property rather than merely a colour adjective.
Compare my reasoning
- Consider the light's frequency distribution.
- The ideal source has one visible frequency.
- That is the monochromatic definition.
Visible light of a single frequency.
Look for these in your work
- State the frequency criterion.
- Avoid equating every colour label with perfect monochromaticity.
Compare white and monochromatic input through a dispersive prism.
Give me a hint
Ask how many frequency components are available to separate.
Compare my reasoning
- White input includes different visible frequencies.
- Those components can emerge with different deviations.
- A single-frequency input has one corresponding deviation rather than the full spectrum.
White can form a separated spectrum; monochromatic light can be deviated without a full spectrum.
Look for these in your work
- Distinguish refraction from dispersion.
- Retain the input distribution.
For f = 6.0 × 10¹⁴ Hz compare wavelengths at speeds 3.0 × 10⁸ and 2.0 × 10⁸ m/s.
Give me a hint
Use λ = v/f at the same frequency.
Compare my reasoning
- The first wavelength is 5.0 × 10⁻⁷ m = 500 nm.
- The second is 3.333… × 10⁻⁷ m ≈ 333 nm.
- The light remains single-frequency despite the wavelength change.
500 nm → approximately 333 nm; frequency unchanged.
Look for these in your work
- Use consistent units.
- Retain monochromatic frequency status.
Common mistakes
- Monochromatic means a single frequency.
- A colour name alone is insufficient proof of an exact frequency distribution.
- Changing wavelength across a material does not require changing frequency.
What can you explain now?
Is a single-frequency beam still monochromatic after ordinary stationary refraction?
Compare with the explanation
Yes.
The frequency remains the same even though speed and material wavelength change.
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Make it stick
Define monochromatic light and explain how single-frequency refraction differs from white-light dispersion.
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