Coloured Light C9.3

A colour filter transmits only its own colour; all other wavelengths are absorbed (colour subtraction).
Viewing objects through a filter:
- If the object’s reflected colour matches the filter, it appears that colour.
- If not, no light passes → object looks black.
White light = red + green + blue (RGB).
When a material absorbs one or more RGB components, the remaining reflected components determine the observed colour.
- Example: absorbs red → reflects blue + green → appears cyan.
Black objects: absorb (nearly) all incident light; reflect none.

Primary light colours: red, green, blue (RGB).
Secondary (additive) colours:
- \text{red} + \text{green} \rightarrow \text{yellow}
- \text{red} + \text{blue} \rightarrow \text{magenta}
- \text{green} + \text{blue} \rightarrow \text{cyan}
\text{red} + \text{green} + \text{blue} \rightarrow \text{white}
TVs & screens vary RGB intensities to create full colour range.

Dispersion: prism or diffraction grating splits white light into spectrum due to wavelength-dependent refraction.
Order of colours (low to high refraction): \text{red} \rightarrow \text{orange} \rightarrow \text{yellow} \rightarrow \text{green} \rightarrow \text{blue} \rightarrow \text{indigo} \rightarrow \text{violet} (ROYGBIV).
Violet refracts the most; red the least.

Rough surfaces/atmospheric particles scatter incident light.
Degree of scattering inversely related to wavelength:
- Violet > blue > … > red (least).
Daytime: greater scattering of shorter wavelengths → sky appears blue.
Sunrise/sunset: light path longer; blue scattered away, more red/orange reach eyes.