Order visible colours by wavelength and frequency
Read the traditional colour sequence in both directions and explain the inverse wave relationship.
Core and Extended; Physics 0625, 2026–2028. Follow the selected Light section; ray diagrams use an ideal thin-lens or plane-boundary model. Written work is self-checked.
Before you begin
- Use v = fλ.
- Recognise white light's continuous visible spectrum.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At a common speed, how do frequency and wavelength vary?
What you will learn
- Know the traditional seven colours in order.
- Order them by frequency and wavelength in a common region.
Know the sequence and what increases
The traditional visible sequence is red, orange, yellow, green, blue, indigo, violet. From red toward violet, frequency increases and vacuum wavelength decreases. In the reverse violet-to-red order, vacuum wavelength increases and frequency decreases. State the quantity and direction when giving an ordered list.
Indigo is one traditional name within a continuous transition rather than a uniquely separated band. Individual colour boundaries depend on naming conventions and perception. The syllabus sequence is useful for ordering; it is not a requirement to memorise sharp universal wavelength boundaries for each named colour.
Use the same region for wavelength comparisons
In vacuum all these components use the same speed c₀, so f = c₀/λ gives the inverse ordering. A supplied 700 nm red-end example has f ≈ 4.29 × 10¹⁴ Hz for c₀ = 3.0 × 10⁸ m/s. A supplied 400 nm violet-end example has f = 7.50 × 10¹⁴ Hz. These example wavelengths illustrate the ordering rather than exact colour boundaries.
Frequency remains unchanged on entering stationary glass, but each component's wavelength becomes shorter according to its material speed. Do not compare one vacuum wavelength with another material wavelength while silently assuming a common speed. For the requested visible-spectrum order, the standard vacuum wavelength reference keeps the comparison consistent.
Pause and explain
Which sequence is in increasing vacuum wavelength order?
Worked example
List the traditional colours in increasing frequency order.
Show the worked solution
- Start at the red low-frequency end.
- Continue orange, yellow, green, blue and indigo.
- Finish at violet, the high-frequency end.
Answer Red, orange, yellow, green, blue, indigo, violet.
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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.
List the traditional colours in decreasing vacuum wavelength order.
Give me a hint
Begin at the longest-wavelength end.
Compare my reasoning
- Red is the long-wavelength end.
- Continue orange, yellow, green, blue and indigo.
- Finish at violet, the short-wavelength end.
Red, orange, yellow, green, blue, indigo, violet.
Look for these in your work
- State the wavelength direction.
- Retain all seven traditional names.
List them in decreasing frequency order.
Give me a hint
Begin at the high-frequency violet end.
Compare my reasoning
- Start violet and indigo.
- Continue blue, green, yellow and orange.
- Finish red.
Violet, indigo, blue, green, yellow, orange, red.
Look for these in your work
- Reverse the correct sequence.
- Keep frequency separate from wavelength.
For supplied vacuum wavelengths 700 and 400 nm, calculate frequencies using 3.0 × 10⁸ m/s.
Give me a hint
Convert nanometres to metres before using f = c₀/λ.
Compare my reasoning
- 700 nm = 7.00 × 10⁻⁷ m and 400 nm = 4.00 × 10⁻⁷ m.
- The frequencies are about 4.29 × 10¹⁴ and 7.50 × 10¹⁴ Hz.
- The shorter wavelength has the higher frequency.
Approximately 4.29 × 10¹⁴ Hz and 7.50 × 10¹⁴ Hz.
Look for these in your work
- Convert the prefixes.
- Check the inverse ordering.
Common mistakes
- Increasing frequency means decreasing vacuum wavelength.
- Name the ordering quantity.
- The traditional names do not create sharp separated spectral bands.
What can you explain now?
Which has the longer vacuum wavelength, red or violet?
Compare with the explanation
Red.
At the same vacuum speed, the lower frequency has the longer wavelength.
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Make it stick
Give both visible-spectrum orderings and explain them using a common speed and v = fλ.
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