Adding coloured lights
The three primary colours of light are red, green and blue (RGB). Addition of light means mixing coloured lights so their colours add together and the result is brighter. Mixing two primary lights gives a secondary colour. Mixing all three primary lights gives white light.
Key idea
red + green = yellow · green + blue = cyan · blue + red = magenta · red + green + blue = white
How we see colour
An object has colour because of the light it reflects. A red apple reflects red light and absorbs the other colours. A white object reflects all colours; a black object absorbs all colours, so little light comes back.
Subtracting light with pigments
Subtraction of light happens with paints, dyes and filters. A coloured filter or pigment absorbs some colours and lets others through or reflects them. For example, a red filter only lets red light pass and absorbs the rest. This is why mixing many paints together makes a dark, muddy colour — more and more light is absorbed.
Example
Under a red filter, a blue object looks black because there is no blue light left to reflect and the object cannot reflect red.
Remember
- Primary colours of light: red, green, blue.
- Adding all three lights makes white.
- Filters and pigments subtract colours by absorbing them.