Know the traditional seven colours of the visible spectrum in order of frequency and in order of wavelength

3.2.3 Thin Lenses – Visible Spectrum Colours

The Seven Colours of Light

We often see a rainbow when sunlight passes through raindrops. The rainbow shows the seven colours of the visible spectrum in a specific order. Think of it like a traffic light that goes from red to violet.

Order of Frequency (Highest to Lowest)

1. Violet (\$f_{\text{violet}}\$)
2. Indigo (\$f_{\text{indigo}}\$)
3. Blue (\$f_{\text{blue}}\$)
4. Green (\$f_{\text{green}}\$)
5. Yellow (\$f_{\text{yellow}}\$)
6. Orange (\$f_{\text{orange}}\$)
7. Red (\$f_{\text{red}}\$)

Order of Wavelength (Shortest to Longest)

1. Violet (\$\lambda_{\text{violet}} \approx 380\$ nm)
2. Indigo (\$\lambda_{\text{indigo}} \approx 445\$ nm)
3. Blue (\$\lambda_{\text{blue}} \approx 475\$ nm)
4. Green (\$\lambda_{\text{green}} \approx 520\$ nm)
5. Yellow (\$\lambda_{\text{yellow}} \approx 580\$ nm)
6. Orange (\$\lambda_{\text{orange}} \approx 620\$ nm)
7. Red (\$\lambda_{\text{red}} \approx 750\$ nm)

Why the Order Matters

Frequency and wavelength are inversely related: \$f = \frac{c}{\lambda}\$ where \$c\$ is the speed of light. So, higher frequency means shorter wavelength.

Analogy: The Music of Light

Imagine each colour as a musical note. The high notes (violet, indigo) are like the treble clef, while the low notes (red, orange) are like the bass clef. Just as a piano has a range of notes, light has a range of colours.

Exam Tip Box

📝 Exam Tip: When asked for the order of colours, remember the mnemonic “ROY G. BIV” (Red, Orange, Yellow, Green, Blue, Indigo, Violet). For frequency, list them from Violet to Red; for wavelength, list from Violet to Red as well but note that wavelength increases as you move from Violet to Red.

Quick Reference Table

Practice Question

Write the colours in order of decreasing frequency. Then write them in order of increasing wavelength. Check your answers against the table above.