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Mixing colours for use on paper is the opposite of transmitting light. In this case the idea is to take certain colours out of white light so that only the desired colour
is reflected from the surface. That is why mixing paints is called subtractive mixing. All paints, inks and dyes work in this way.
For example, mixing yellow
and blue pigments causes the absorption of all colours but green.
The colours for subtraction are those that absorb red, green and blue light. They are blue–green (cyan), red–blue (magenta) and yellow. Confusion arises when some people call the primary colours used in paints red, yellow and blue (instead of magenta, yellow and cyan). For paints it is always best to think of the colours as red-absorbing, green-absorbing and blue-absorbing. Then you can see that transmitted and reflected light (see: Reflection) are part of the same system.
Colour of light sources
As objects, such as the filament in a light bulb, get hotter, the length of the light waves they give out changes. At a temperature of about 500°C the filament gives out a red light. As the temperature rises, the colour is yellow. Then, at about 5,000°C white light is given out.
A tungsten light bulb (see: Electric light) will reach about 3,000°C and give out a yellowish light. Average daylight from the Sun produces the same light at ground level as an object heated to 4,800°C; but from space the light from the Sun is whiter, appearing similar to the light from an object heated to 5,800°C.
Colour vision
To be able to see in colour, we must have detectors for colour inside our
eyes. These detectors are found
at the back of the eye. They are tiny groups of cells (called cones) that contain chemicals that are sensitive to different kinds of light. Although all cones are sensitive to every kind of light, it is thought that some cones are more sensitive to blue light, some more sensitive to green light and the rest more sensitive to red light.
Complementary colours
Colours that, when added together, produce white. Blues
are complementary to yellows, reds to blue–greens and greens
to red–purples. Newton’s colour circle shows which colours are complementary and can be mixed to produce white. (See also: Colour mixing and Primary colours.)
Concave mirror
 Concave mirror – A concave mirror makes things look bigger; the distortion depends on the amount that the lens is concave. A light placed at the centre of a concave mirror will be turned into a beam.
Concave mirror
Concave mirror
Concave lens
A lens that curves inwards. These lenses cause light rays to spread apart, or diverge. They do not produce a real image, but a virtual (imaginary) image that appears to be in front of the lens. They are reducing lenses.
Concave lens
F
Focal length
 Concave lens – A concave lens produces a smaller virtual image that cannot be focused on a screen. It is used in eyeglasses for correcting the vision of people with myopia.
Concave mirror
A mirror that curves inwards. Light rays, from a source such
as a light bulb, travel outwards in all directions. Sometimes the light needs to be made into a beam, travelling in just one direction. A concave mirror is used to bounce the rays of light travelling away from where they are wanted and turn them into a beam travelling in just one direction. (See also: Lighthouse.)
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