The human eye is sensitive to electromagnetic radiation at a wavelength ranging from 400 to 700 nanometres. This range is visible light in common parlance. When all the wavelengths in the above width are “mixed” together, we get white light. White light with a prism can give all the colors that “produce” it.
However, red, green and blue are the primary colours for humans, because they cannot be produced by mixing other colours, as is the case with the others. If you mix red, green and blue in exactly equal amounts, you get white.
But beyond the basic colours, there are also complementary colours. Complementary colours are obtained by mixing the primary colours in pairs. These are yellow, cyan (blue) and purple (violet), which have the characteristic that if they are mixed in pairs, the primary colours are obtained. For example, while mixing equal amounts of green and red gives us yellow, mixing yellow and purple in equal amounts gives us red.
Let us now see how the colour of bodies is affected by the colour of the light that falls on them. We have a deck of cards, a bunch of blue grapes and a green pepper. We illuminate these three objects successively with white, red, green and blue light.
In the white light we see all three bodies as we expect to see them, the card white, the grapes blue and the pepper green. Now in red light the card reflects all the radiation it receives. In contrast, the other two bodies absorb most of the radiation and reflect little so that they cannot be seen clearly and the dominant colour is black. In the green suit, the card appears green but the symbols on it are black, which means that the green radiation was reflected by the white part of the card but absorbed by the red symbols. The blue grape “reacts” in a similar way when the red light was shone on it, while the pepper keeps its colour, but has some areas that are more intensely green. These areas in white light were glowing. Finally, similar phenomena are observed with blue light. The card of cards is now blue with its symbols black, the grape has its colour, while the pepper is dominated by black with some areas being blue.
The human eye is very sensitive to the nuances of colours and is probably capable of distinguishing 8-12 million shades. Most colours contain some amount of all wavelengths of the visible spectrum. What makes colours different is the distribution of wavelengths in a colour. The wavelengths of a color that dominate ultimately determine the hue. If some wavelengths dominate much more than others, a body color will be intense, while if these wavelengths are roughly equal in quantity, the color will be non-intense, dull. Whether or not a body will have an intense color is determined by the reflectivity of the body and the intensity of the color.
Most of what we've said so far has been about the properties of visible light in relation to certain colours. But the images of our everyday life, the objects we see are the result of the reflection of white light on objects, houses, cars, trees, etc. These bodies do not emit light naturally, but obey a property that says that some of the wavelengths (or some colours in other words) of white light are absorbed by the bodies and some are reflected. A blue car, for example, under sunlight absorbs all colours and reflects blue. This is the way it is “colored” in our eyes.
Colour photographs, a painting and other objects characterised by colour are produced from only four inks in reality. These inks include the three complementary colours, blue, purple and yellow plus black. If these inks are mixed in the appropriate quantities they can do each colour. Theoretically, only the three complementary colours are sufficient, but in order to achieve the right shading each time we need in practice the black as well. In printing a book or magazine, the following procedure is followed. We take a photograph of a body, which is first divided into the three complementary colours (in the appropriate quantities, of course) by some photographic methods or by computer. The three colours are converted into film of the corresponding colour. The film is the agent that allows the object to be printed on paper. So to print an object we need three films plus the black one for better shading.
