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

3.2.3 Thin Lenses

Objective

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

Why Thin Lenses Produce a Spectrum

A converging (convex) thin lens can focus parallel rays of white light to a point called the focal point. Because the refractive index of the lens material varies slightly with wavelength (dispersion), different colours are brought to focus at slightly different distances from the lens. This separation of white light into its constituent colours is the same principle that produces a rainbow in a prism.

Lens Formulae (for reference)

The relationship between object distance \$u\$, image distance \$v\$, and focal length \$f\$ for a thin lens is

\$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}\$

The linear magnification \$m\$ is

\$m = \frac{v}{u} = \frac{h'}{h}\$

where \$h\$ and \$h'\$ are the object and image heights respectively.

Traditional Seven Colours of the \cdot isible Spectrum

The colours are traditionally remembered by the mnemonic “ROY GBV”. They can be ordered either by decreasing wavelength (longest to shortest) or by increasing frequency (lowest to highest).

Order of Wavelength (Longest → Shortest)

1. Red
2. Orange
3. Yellow
4. Green
5. Blue
6. Indigo
7. Violet

Order of Frequency (Lowest → Highest)

1. Violet
2. Indigo
3. Blue
4. Green
5. Yellow
6. Orange
7. Red

Combined Table

Quick Revision Checklist

• Recall the mnemonic “ROY GBV”.
• Remember that wavelength decreases from red to violet, while frequency increases in the same direction.
• Understand that a thin convex lens separates these colours because of dispersion.
• Be able to write the lens formulae and explain how the focal length varies slightly with colour.