9.1 Paints and pigments

Pigments and paints

We have learnt that white light is a combination of all the colours of the visible spectrum and that each colour of light is related to a different frequency. But what gives everyday objects around us their different colours?

Pigments are substances which give an object its colour by absorbing certain frequencies of light and reflecting other frequencies. For example, a red pigment absorbs all colours of light except red which it reflects. Paints and inks contain pigments which gives the paints and inks different colours.

Colour of opaque objects

Objects which you cannot see through (i.e. they are not transparent) are called opaque . Examples of some opaque objects are metals, wood and bricks. The colour of an opaque object is determined by the colours (therefore frequencies ) of light which it reflects . For example, when white light strikes a blue opaque object such as a ruler, the ruler will absorb all frequencies of light except blue, which will be reflected. The reflected blue light is the light which makes it into our eyes and therefore the object will appear blue.

Opaque objects which appear white do not absorb any light. They reflect all the frequencies. Black opaque objects absorb all frequencies of light. They do not reflect at all and therefore appear to have no colour.

Colour of transparent objects

If an object is transparent it means that you can see through it. For example, glass, clean water and some clear plastics are transparent. The colour of a transparent object is determined by the colours (frequencies) of light which it transmits (allows to pass through it). For example, a cup made of green glass will appear green because it absorbs all the other frequencies of light except green, which it transmits. This is the light which we receive in our eyes and the object appears green.

Pigment primary colours

The primary pigments and paints are cyan , magenta and yellow . When pigments or paints of these three colours are mixed together in equal amounts they produce black . Any other colour of paint can be made by mixing the primary pigments together in different quantities. The primary pigments are related to the primary colours of light in the following way:

End of chapter exercises

1. Calculate the wavelength of light which has a frequency of $570\times {10}^{12}$ Hz.
2. Calculate the frequency of light which has a wavelength of 580 nm.
3. Complete the following sentence: When white light is dispersed by a prism, light of the colour ? is refracted the most and light of colour ? is refracted the least.
4. What are the two types of photoreceptor found in the retina of the human eye called and which type is sensitive to colours?
5. What color do the following shirts appear to the human eye when the lights in a room are turned off and the room is completely dark?
   1. red shirt
   2. blue shirt
   3. green shirt
6. Two light bulbs, each of a different colour, shine on a sheet of white paper. Each light bulb can be a primary colour of light - red, green, and blue. Depending on which primary colour of light is used, the paper will appear a different color. What colour will the paper appear if the lights are:
   1. red and blue?
   2. red and green?
   3. green and blue?
7. Match the primary colour of light on the left to its complementary colour on the right:

   Column A	Column B
   red	yellow
   green	cyan
   blue	magenta

8. Which combination of colours of light gives magenta?
   1. red and yellow
   2. green and red
   3. blue and cyan
   4. blue and red
9. Which combination of colours of light gives cyan?
   1. yellow and red
   2. green and blue
   3. blue and magenta
   4. blue and red
10. If yellow light falls on an object whose pigment absorbs green light, what colour will the object appear?
11. If yellow light falls on a blue pigment, what colour will it appear?